ClutterEffect is an abstract class that should be used to apply effects
on generic actors.
The ClutterEffect class just defines what an effect should implement; it
could be defined as an interface, but we might want to add some default
behavior dependent on the internal state at a later point.
The effect API applies to any actor, so we need to provide a way to
assign an effect to an actor, and let ClutterActor call the Effect
methods during the paint sequence.
Once an effect is attached to an actor we will perform the paint in this
order:
• Effect::pre_paint()
• Actor::paint signal emission
• Effect::post_paint()
Since an effect might collide with the Shader class, we either allow a
shader or an effect for the time being.
It's valid C to declare a function omitting it prototype, but it seems
to be a good practise to always declare a function with its
corresponding prototype.
While this is totally fine (None is 0L and, in the pointer context, will
be converted in the right internal NULL representation, which could be a
value with some bits to 1), I believe it's clearer to use NULL instead
of None when we talk about pointers.
This adds a boolean "pick-with-alpha" property to ClutterTexture and when
true, it will use the textures alpha channel to define the actors shape when
picking.
Users should be aware that it's a bit more expensive to pick textures like
this (so probably best not to blindly enable it on *all* your textures)
since it implies rasterizing the texture during picking whereas we would
otherwise just send a solid filled quad to the GPU. It will also interrupt
the internal batching of geometry for pick renders which can otherwise often
be done in a single draw call.
The DragAction should, by default, drag the actor to which it has been
applied, instead of delegating what to do to the developer. If custom
code need to override it, g_signal_stop_emission_by_name() can be called
to stop the default handler to ever running.
The drawing code from test_invalid_texture_layers which draws a
rectangle, a polygon and a vertex buffer has been split out to
separate function. test_using_all_layers now also uses this so that it
will also test the other two primitives. This causes the test to fail
when all of the layers are drawn using a vertex buffer.
http://bugzilla.openedhand.com/show_bug.cgi?id=2132
* wip/state-machine:
Do not use wildcards in test-state
script: Implement State deserialization
state: added a "target-state" property
state: documented data structures
Add State interactive tests to the ignore file
state: Documentation and introspection annotation fixes
state: Minor coding style fixes
state: Clean up the header's documentation
state: Constify StateKey accessors
Do not include clutter.h from a Clutter header file
state-machine: made clutter_state_change take a boolean animate argument
state-machine: use clutter_timeline_get_progress
state-machine: add completed signal
state machine: added state machine
Conflicts:
.gitignore
* wip/constraints: (24 commits)
Add the Cogl API reference to the fixxref extra directories
Document the internal MetaGroup class
Remove the construct-only flag from ActorMeta:name
doc: Remove gtk-doc annotations from the json-glib copy
doc: Fix parameter documentation
Add named modifiers for Action and Constraint
Remove a redundant animation
Set the stage resizable in test-constraints
Use a 9 grid for the constraints test
Miscellaneous documentation fixes
docs: Document animating action and constraint properties
docs: Document BindConstraint and AlignConstraint
constraint: Rename BindConstraint:bind-axis
constraints: Add AlignConstraint
tests: Add a constraints interactive test
constraint: Add BindConstraint
actor: Implement Animatable
animation: Use the new Animatable API for custom properties
animatable: Add custom properties to Animatable
constraint: Add ClutterConstraint base class
...
Conflicts:
configure.ac
The ClutterActor API should have modifier methods for adding, removing
and retrieving Actions and Constraints using the ClutterActorMeta:name
property - mostly, for convenience.
To demonstrate that constraints can be used to reposition actors in a
resizable stage, set the :user-resizable property to TRUE, and spend the
next 15 minutes playing with the stage size.
AlignConstraint is a simple constraint that keeps an actor's position
aligned to the width or height of another actor, multiplied by an
alignment factor.
GLES 2 doesn't have GL_MAX_TEXTURE_UNITS. Instead the cogl backend
uses GL_MAX_TEXTURE_IMAGE_UNITS with a maximum limit of 16. The same
restriction is now used in the test.
DragAction is an Action sub-class that provides dragging capabilities to
any actor. DragAction has:
• drag-begin, drag-motion and drag-end signals, relaying the event
information like coordinates, button and modifiers to user code;
• drag-threshold property, for delaying the drag start by a given
amount of pixels;
• drag-handle property, to allow using other actors as the drag
handle.
• drag-axis property, to allow constraining the dragging to a specific
axis.
An interactive test demonstrating the various features is also provided.
This adds a simple test for ClutterCairoTexture that draws two
rectangles to the cairo surface in an idle callback and then verifies
that they appeared at the right colours in the paint callback. If that
succeeds then the second time the idle callback is invoked it will
replace one of the rectangles with a sub region update and the
following paint callback will again verify the rectangles.
Instead of asking gtester to run ./test-conformance directly we now tell
it to run a list of wrapper scripts. This results in each test being
spawned in a separate process avoiding leakage of state between tests
which has been a big problem with the conformance tests for quite a
while now.
Otherwise it seems that rounding errors will cause the fragments at
the edge of the quad to blend with neighbouring quarters of the
texture which cause the test to fail.
A few of the tests connected to the paint signal but never
disconnected it. Most of these handlers had a call to g_main_quit in
them which meant that it could sometimes cause subsequent tests to
exit after the first frame is painted. Most of the tests don't
validate any of the results until after a couple of frames have been
rendered so this ended up skipping out the test entirely.
To workaround this the test setup function now disconnects all
handlers for the paint signal on the default stage before the test is
run.
The on_paint function for test-cogl-readpixels tries to temporarily
set the projection, modelview and viewport to its own values. However
it was never restoring the saved values so it could affect the results
of subsequent tests.
This adds a test which creates a material using the maximum number of
layers. All of the layers are assigned a white texture except the last
which is set to red. The default combine mode is used for all of the
layers so the final fragment should end up red.
Currently Cogl doesn't provide a way to query the maximum number of
layers so it just uses glGetIntegerv instead. This might cause
problems on GLES 2 because that adds additional restrictions on the
number of layers.
http://bugzilla.openedhand.com/show_bug.cgi?id=2064
This changes the original tests so that it splits the original path
into two sub paths. When adding a new block to the copied path it also
adds another sub path. This further stresses the path copying
mechanism and exposes a bug.
It also tests intersections by drawing a self-intersecting path and a
path with two sub-paths that overlap. Where the path overlaps it
should be inverted.
This renders a texture using different combinations of wrap modes for
the s and t coordinates and then verifies that the expected wrapping
is acheived. The texture is drawn using rectangles, polygons and
vbos. There is also code to test a rectangle using an atlased texture
(which should test the manual repeating) however the validation for
this is currently disabled because it doesn't work.
http://bugzilla.openedhand.com/show_bug.cgi?id=2063
This tests various paths drawing rectangles and verifies that the
expected pixels are filled in. Some of the paths are drawn by copying
an existing path and modifying it which should test the copy-on-write
code.
This updates test-cogl-offscreen to give an example of setting up a
CoglFramebuffer with a Clutter style 2D coordinate system with (0,0) top
left and (framebuffer_width, framebuffer_height) bottom right.
test-cogl-tex-polygon creates a separate material so that it can set
the texture filters. However in the paint functions that use
cogl_polygon it was calling cogl_set_source_texture which replaces the
material and ends up always using GL_LINEAR. This patch makes the
paint functions assume the correct source is set up instead of trying
to select a new source.
Added new "homogeneous" mode to ClutterBoxLayout, that makes layout children
get all the same size.
This is heavily inspired in the "homogeneous" attribute available in GtkBox,
but simplified as we don't have padding nor borders in box layout, only
spacing.
Also added to test-box-layout a key to set/unset homogeneous mode.
* Coding style fixes.
* Added proper test for homogeneous mode in box layout.
* Fix in homogeneous mode.
http://bugzilla.openedhand.com/show_bug.cgi?id=2034
Signed-off-by: Emmanuele Bassi <ebassi@linux.intel.com>
The test creates a GL_TEXTURE_RECTANGLE_ARB texture using
cogl_texture_new_from_foreign and confirms that rendering it works
correctly. If the rectangle texture extension isn't available then
this test always succeeds.
http://bugzilla.openedhand.com/show_bug.cgi?id=2015
The FlowLayout fails to provide a preferred size in case no sizing is
specified on one axis. It should, instead, have the preferred size of
the sum of its children, depending on the orientation property.
http://bugzilla.openedhand.com/show_bug.cgi?id=2013
cogl_read_pixels() no longer asserts that the format passed in is
RGBA_8888 but instead accepts any format. The appropriate GL enums for
the format are passed to glReadPixels so OpenGL should be perform a
conversion if neccessary.
It currently assumes glReadPixels will always give us premultiplied
data. This will usually be correct because the result of the default
blending operations for Cogl ends up with premultiplied data in the
framebuffer. However it is possible for the framebuffer to be in
whatever format depending on what CoglMaterial is used to render to
it. Eventually we may want to add a way for an application to inform
Cogl that the framebuffer is not premultiplied in case it is being
used for some special purpose.
If the requested format is not premultiplied then Cogl will convert
it. The tests have been changed to read the data as premultiplied so
that they won't be affected by the conversion. Picking in Clutter has
been changed to use COGL_PIXEL_FORMAT_RGB_888 because it doesn't need
the alpha component. clutter_stage_read_pixels is left unchanged
because the application can't specify a format for that so it seems to
make most sense to store unpremultiplied values.
http://bugzilla.openedhand.com/show_bug.cgi?id=1959
Allow a ClutterModel to be constructed through the ClutterScript API.
Currently this allows a model to be generated like like this:
{
"id" : "test-model",
"type" : "ClutterListModel",
"columns" : [
[ "text-column", "gchararray" ],
[ "int-column", "gint" ],
[ "actor-column", "ClutterRectangle" ]
]
}
where 'columns' is an array containing arrays of column-name,
column-type pairs.
http://bugzilla.openedhand.com/show_bug.cgi?id=2007
The adds tests for the remaining layer combine functions, the 1 minus
value operator and the TEXTURE_N source. Note however that Cogl
currently fails when parsing a TEXTURE_N source so the test is
commented out.
We need to make the Stage set the MAPPED flag on itself if we want to
verify the MAPPED state. That was always the case - it just worked
before because the Stage was shown at least once.
It was passing the number of vertices to
cogl_vertex_buffer_draw_elements but instead it should take the
maximum index which would be the number of vertices minus one. This
was causing errors to be reported with the checks filterset of Bugle.
If an actor is lying on the border of the Stage it might miss the LEAVE
event when the pointer of a device leaves the Stage window. Since the
backend is unsetting the Stage back pointer on the InputDevice we can
queue the emission of a LEAVE event on the pointer actor as well.
http://bugzilla.moblin.org/show_bug.cgi?id=9677
Since all conformance tests share the same state we should not touch
stuff like the stage size; sharing is already fairly complex and adds a
lot of caveats on the implementation of a conformance test unit, and if
we make tests influence later ones then we might slip in bugs or false
negatives - thus defeating the whole point of a conformance test suite.
The g_assert_cmpint() macro prints out not just the assertion condition
but also the assertion contents; this is useful to catch wrong values
without incrementing the verbosity of the test itself.
Since the "internal" state is global, it will leak onto actors that you
didn't intend for it to, because it applies not just to the actors you
create, but also to any actors *they* create. Eg, if you have a dialog
box class, you might push/pop_internal around creating its buttons, so
that those buttons get marked as internal to the dialog box. But
ctx->internal_child will still be set during the *button*'s constructor
as well, and so, eg, the label and icon inside the button actor will
*also* be marked as internal children, even if that isn't what the
button class wanted.
The least intrusive change at this point is to make push_internal() and
pop_internal() two methods of the Actor class, and take a ClutterActor
pointer as the argument - thus moving the locality of the internal_child
counter to the Actor itself.
http://bugzilla.openedhand.com/show_bug.cgi?id=1990
When creating a Cogl sub-texture, if the full texture is also a sub
texture it will now just offset the x and y and reference the full
texture instead. This avoids one level of indirection when rendering
the texture which reduces the chances of getting rounding errors in
the calculations.
The test was calling g_object_get to fetch the "opacity-start" property
(unsigned int) into a guint8 local variable. It's a bit of a mean trap
given that the getter function returns guint8 values so this also adds a
comment explaining what's going on.
* animator-parser:
docs: Describe the Animation definition syntax
animator: Provide a ClutterScript parser
animator: Allow retrieving type property type from a key
script: Use a node when resolving an animation mode
The whole point of having the Animator class is that the developer can
describe a complex animation using ClutterScript. Hence, ClutterAnimator
should hook into the Script machinery and parse a specific description
format for its keys.
ClutterAnimator is a class for managing the animation of multiple
properties of multiple actors over time with keyframing of values.
The Animator class is meant to be used to effectively describe
animations using the ClutterScript definition format, and to construct
complex implicit animations from the ground up.
Signed-off-by: Emmanuele Bassi <ebassi@linux.intel.com>
This adds three new texture backends.
- CoglTexture2D: This is a trimmed down version of CoglTexture2DSliced
which only supports a single texture and only works with the
GL_TEXTURE_2D target. The code is a lot simpler so it has a less
overheads than dealing with slices. Cogl will use this wherever
possible.
- CoglSubTexture: This is used to get a CoglHandle to represent a
subregion of another texture. The texture can be used as if it was a
standalone texture but it does not need to copy the resources.
- CoglAtlasTexture: This collects RGB and RGBA textures into a single
GL texture with the aim of reducing texture state changes and
increasing batching. The backend will try to manage the atlas and
may move the textures around to close gaps in the texture. By
default all textures will be placed in the atlas.
When running tests under Valgrind it would be useful to pass a
suppression file for the known one-off allocations done by Clutter
and by its dependencies. This trims the output of Valgrind and
improves the ability to actually spot leaks.
The coverage of the Behaviour sub-classes is currently abysmal. An
initial test suite for Behaviours should at least verify that the
accessors and the constructors are doing the right thing.
This initial test suite just verifies the BehaviourOpacity sub-class,
but it already bumps up the overall coverage by 2%.
* device-manager: (37 commits)
x11: Re-enable XI1 extension keyboards
x11: Always handle core device events before XI events
docs: Documentation fixes for DeviceManager
device-manager: Fix the signals definition
docs: Add sections for InputDevice and DeviceManager
docs: Add clutter_input_device_get_device_name()
tests: Print out the device details on motion
Always register core devices
device: Remove unused is_default member
win32: Experimental implementation of device support
tests: Print the device name, as well as its Id
x11: Fill out the :name property of the InputDevices
device: Add the :name property to InputDevice
x11: Store core devices on the X11 Backend singleton
device: Unset the cursor actor when leaving the stage
device: Add pointer actor getter
x11: Discard the LeaveNotify for off-stage ButtonRelease
device: Do not overwrite the stage for an InputDevice
event: Off-stage button releases have a click count of 1
event: Scroll events do not have click count
...
It's very useful to see the actual number the reference value is
compared too when the test fails. GTest has g_assert_cmp$type()
functions for that, so make good use of them.
A small doubt has risen about the use of CoglTextureUnit in materials:
will texture matrices still work if we have several materials, each of
them having at texture on the same texture unit? The answer is yes!
test-cogl-multitexture has been extended to use 2 materials with about
the same setup except a little difference: the texture matrices for the
lightmaps rotate in opposite directions.
While at it, changed the rotation behaviour by an implicit animation
with a small additional bonus bling.
The extension keyboard support in XInput 1.x is hopelessly broken.
Nevertheless, it's possible to use some bits of it, as we prefer the
core keyboard events to the XInput events, thus at least having proper
handling for X11 key events on the Stage window.
• The enter/leave event line should take into account the case where
the related field is set to NULL (meaning entering from off-stage
and leaving the stage).
• The ButtonRelease line shows the click count but uses the button; the
button *and* the click count should be displayed for both ButtonPress
and ButtonRelease, to verify they match.
The output of test-events is a bit of a mess; this patch should clean
it up a little bit - at least enough for it to be useful again during
visual inspection.
Otherwise the paint handler will still be run for the subsequent
tests. This ends up writing to the ‘state’ variable which used to be
on the stack so it will end up corrupting some stack variable. This
was causing test-cogl-premult to fail.
The sub texture backend doesn't work well as a completely general
texture backend because for example when rendering with cogl_polygon
it needs to be able to tranform arbitrary texture coordinates without
reference to the other coordintes. This can't be done when the texture
coordinates are a multiple of one because sometimes the coordinate
should represent the left or top edge and sometimes it should
represent the bottom or top edge. For example if the s coordinates are
0 and 1 then 1 represents the right edge but if they are 1 and 2 then
1 represents the left edge.
Instead the sub-textures are now documented not to support coordinates
outside the range [0,1]. The coordinates for the sub-region are now
represented as integers as this helps avoid rounding issues. The
region can no longer be a super-region of the texture as this
simplifies the code quite a lot.
There are two new texture virtual functions:
transform_quad_coords_to_gl - This transforms two pairs of coordinates
representing a quad. It will return FALSE if the coordinates can
not be transformed. The sub texture backend uses this to detect
coordinates that require repeating which causes cogl-primitives
to use manual repeating.
ensure_non_quad_rendering - This is used in cogl_polygon and
cogl_vertex_buffer to inform the texture backend that
transform_quad_to_gl is going to be used. The atlas backend
migrates the texture out of the atlas when it hits this.
This adds a test which renders a texture into a 1x1 pixel quad with
and without filters that use mipmaps. The pixel without mipmaps will
be one of the colors from the texture and the one with will be the
average of all the pixels in the texture.
If a user supplied multiple groups of texture coordinates with
cogl_rectangle_with_multitexture_coords() then we would repeatedly log only
the first group in the journal. This fixes that bug and adds a conformance
test to verify the fix.
Thanks to Gord Allott for reporting this bug.
The test-text "fake scrolling" code stopped working somewhere between
0.8 and 0.9, with the new layout code. Instead of the ::cursor-event
signal it should use an approach similar to the Scrollable interface in
the Moblin User Experience toolkit.
Right now, it makes sense to ignore this code entirely.
* animate-layout-manager:
layout-manager: Document the animation support
layout-manager: Rewind the timeline in begin_animation()
box-layout: Remove the allocations hash table
docs: Clean up the README file
layout: Let begin_animation() return the Alpha
box-layout: Add knobs for controlling animations
box-layout: Animate layout properties
layout: Add animation support to LayoutManager
Add ActorBox animation methods
* stage-use-alpha:
tests: Use accessor methods for :use-alpha
stage: Add accessors for :use-alpha
tests: Allow setting the stage opacity in test-paint-wrapper
stage: Premultiply the stage color
stage: Composite the opacity with the alpha channel
glx: Always request an ARGB visual
stage: Add :use-alpha property
materials: Get the right blend function for alpha
ClutterLayoutManager does not have any state associated with it, and
defers all the state to its sub-classes.
The BoxLayout is thus in charge of controlling:
• whether or not animations should be used
• the duration of the animation
• the easing mode of the animation
By adding three new properties:
• ClutterBoxLayout:use-animations
• ClutterBoxLayout:easing-duration
• ClutterBoxLayout:easing-mode
And their relative accessors pairs we can make BoxLayout decide whether
or not, and with which parameters, call the begin_animation() method of
ClutterLayoutManager.
The test-box-layout has been modified to reflect this new functionality,
by checking the key-press event for the 'a' key symbol to toggle the use
of animations.
This tests creating a sub texture from a larger texture using various
different texture coordinates. It also tries to read back the texture
data using cogl_texture_get_data.
* text-direction:
docs: Add text-direction accessors
Set the default language on the Pango context
actor: Set text direction on parenting
tests: Display the index inside text-box-layout
box-layout: Honour :text-direction
text: Dirty layout cache on text direction changes
actor: Add :text-direction property
Use the newly added ClutterTextDirection enumeration
Add ClutterTextDirection enumeration
This test did not open redhand.png as argc/argv should be handled
normally (argv[0] being the name of the exectutable).
Reviewed-by: Emmanuele Bassi <ebassi@linux.intel.com>
Some tests can be given extra arguments. The test-interactive dispatch
mechanism handles that but the small shell scripts around the wrapper
do not forward the arguments to the wrapper.
Reviewed-by: Emmanuele Bassi <ebassi@linux.intel.com>
cogl_push_draw_buffer, cogl_set_draw_buffer and cogl_pop_draw_buffer are now
deprecated and new code should use the new cogl_framebuffer_* API instead.
Code that previously did:
cogl_push_draw_buffer ();
cogl_set_draw_buffer (COGL_OFFSCREEN_BUFFER, buffer);
/* draw */
cogl_pop_draw_buffer ();
should now be re-written as:
cogl_push_framebuffer (buffer);
/* draw */
cogl_pop_framebuffer ();
As can be seen from the example above the rename has been used as an
opportunity to remove the redundant target argument from
cogl_set_draw_buffer; it now only takes one call to redirect to an offscreen
buffer, and finally the term framebuffer may be a bit more familiar to
anyone coming from an OpenGL background.
Since the rectangles packed inside the BoxLayout interactive test
have random colors it's not easy to verify the ordering. By using
the CoglPango API to print out the index in the middle of each
rectangle.
These changes caused the test to try to load redhand.png but this no
longer works since the TESTS_DATADIR changes in 0b4899ef23. The only
use of the hand is commented out anyway so it looks like the changes
were intended for temporary debugging.
cogl_clip_push, and cogl_clip_push_window_rect which are now deprecated were
used in various places internally so this just switches to using the
replacement functions.
cogl_clip_push() which accepts a rectangle in model space shouldn't have
been defined to take x,y,width,height arguments because this isn't consistant
with other Cogl API dealing with model space rectangles. If you are using a
coordinate system with the origin at the center and the y+ extending up,
then x,y,width,height isn't as natural as (x0,y0)(x1,y1). This API has
now been replace with cogl_clip_push_rectangle()
(As a general note: the Cogl API should only use the x,y,width,height style
when the appropriate coordinate space is defined by Cogl to have a top left
origin. E.g. window coordinates, or potentially texture coordinates)
cogl_clip_push_window_rect() shouldn't have been defined to take float
arguments since we only clip with integral pixel precision. We also
shouldn't have abbreviated "rectangle". This API has been replaced with
cogl_clip_push_window_rectangle()
cogl_clip_ensure() wasn't documented at all in Clutter 1.0 and probably
no one even knew it existed. This API isn't useful, and so it's now
deprecated. If no one complains we may remove the API altogether for
Clutter 1.2.
cogl_clip_stack_save() and cogl_clip_stack_restore() were originally added
to allow us to save/restore the clip when switching to/from offscreen
rendering. Now that offscreen draw buffers are defined to own their clip
state and the state will be automatically saved and restored this API is now
redundant and so deprecated.
The build for interactive tests creates symbolic links for the data
under tests/data; it also uses symbolic links for creating "binaries"
for each interactive test. This is less than ideal, though.
Instead, the tests should build a path to the data files by using
a pre-processor define like TESTS_DATADIR; both g_build_filename() and
pre-processor string concatenation can be used to generate a valid
file name with the full path to the files.
The build system should also create wrapper scripts, just like we
do inside the conformance test suite, to be able to launch single
tests.
The ClutterScript parser needs to be extended to parse child properties
and apply them after an actor has been added to a container. In order to
distinguish child properties from regular GObject properties we can use
the "child::" prefix, e.g.:
{
"type" : "ClutterRectangle",
"id" : "child-01",
"child::has-focus" : true,
...
}
Parsing child properties can be deferred to the ClutterScriptable
interface, just like regular properties.
All the ClutterColor parsing rules should be coalesced inside
clutter_script_parse_color(): object, array and string notations
are the canonical ways of defining a ClutterColor inside a
ClutterScript definition. Having a single function in charge of
the parsing cleans up the code.
ClutterScript is a very complicated piece of machinery, with a
parser that has custom variations on top of the basic JSON
format; it could also be extended in the future, so if we don't
want to introduce regressions or break existing ClutterScript
definitions, we'd better have a conformance test suite.
The units under the conformance test suite should be able to use
external files. Linking the files in tests/conform like the
interactive tests do seems like a hack piled on top of a hack, so
instead we should provide a programmatic way for a conformance
test unit to get the full path of a file, regardless of where the
tests/data directory is.
We can use a define to get the full path of tests/data and then
a function using g_build_filename() to construct the path to the
file we want.
This test tried to do too much, and I can't remember the last time I saw this
test work.
It no longer tries to create a texture from an offscreen actor and it no
longer tries to use shaders.
It does though show that chaining of clutter_texture_new_from_actor now
works, and that animating the source actor is reflected in textures created
from it.
When run you should see three actors:
- on the left is the pristine source actor rotating around the y-axis
- in the middle is the first texture created from the source actor
- and on the right a texture created from the middle actor
Note: the somewhat strange bobbing of the middle and right textures is
actually correct given how it was decided long ago to map the transformed
(to screen space) allocation of the source actor to the texture. When the
hand spins around the perspective projection of the top of the hand results
in the origin of the texture bobbing up to a higher stage position, but the
position of the textures is fixed. This design also means we end up
reallocating our offscreen draw buffer every frame that the actors
transformed size changes, which isn't ideal.
Since offscreen rendering is internally forced to be upside down Cogl
needs to reverse the glFrontFace winding order so as not to interfere
with the use of cogl_set_backface_culling_enabled()
This ensures we test that mechanism.
Mostly this was written to verify that we don't flip the data read back from
an offscreen draw buffer. (since all offscreen rendering is done upside
down)
