Instead of flushing the journal whenever the current framebuffer on a
context is changed it is now flushed whenever the framebuffer is about
to be destroyed instead. To do this it implements a custom unref
function which detects when there is going to be exactly one reference
on the framebuffer and then flushes its journal. The journal now
always has a reference on the framebuffer whenever it is non-empty.
That means the unref will only cause a flush if the only thing keeping
the framebuffer alive is the entries in the journal.
Reviewed-by: Robert Bragg <robert@linux.intel.com>
Unlike in GObject the type number for a CoglObject is entirely an
internal implementation detail so there is no need to make a GQuark to
make it safe to export out of the library. Instead we can just
directly use a fixed pointer address as the identifier for the type.
This patch makes it use the address of the class struct of the
identifier. This should make it faster to do type checks because it
does not need to call a function every time it wants to get the type
number.
Reviewed-by: Robert Bragg <robert@linux.intel.com>
Previously flushing the matrices was performed as part of the
framebuffer state. When on GLES2 this matrix flushing is actually
diverted so that it only keeps a reference to the intended matrix
stack. This is necessary because on GLES2 there are no builtin
uniforms so it can't actually flush the matrices until the program for
the pipeline is generated. When the matrices are flushed it would
store the age of modifications on the matrix stack so that it could
detect when the matrix hasn't changed and avoid flushing it.
This patch changes it so that the pipeline is responsible for flushing
the matrices even when we are using the GL builtins. The same
mechanism for detecting unmodified matrix stacks is used in all
cases. There is a new CoglMatrixStackCache type which is used to store
a reference to the intended matrix stack along with its last flushed
age. There are now two of these attached to the CoglContext to track
the flushed state for the global matrix builtins and also two for each
glsl progend program state to track the flushed state for a
program. The framebuffer matrix flush now just updates the intended
matrix stacks without actually trying to flush.
When a vertex snippet is attached to the pipeline, the GLSL vertend
will now avoid using the projection matrix to flip the rendering. This
is necessary because any vertex snippet may cause the projection
matrix not to be used. Instead the flip is done as a forced final step
by multiplying cogl_position_out by a vec4 uniform. This uniform is
updated as part of the progend pre_paint depending on whether the
framebuffer is offscreen or not.
Reviewed-by: Robert Bragg <robert@linux.intel.com>
_cogl_framebuffer_flush_state needs to handle the case where
ctx->current_draw_buffer is NULL because this will be set in the
destructor for CoglFramebuffer if the framebuffer being destroyed is
the current framebuffer. This patch just makes it assume all state has
changed in that case.
Reviewed-by: Robert Bragg <robert@linux.intel.com>
We should not be deciding whether we need to really update the GL face
winding state at the point where a new framebuffer has been pushed, we
should be waiting until we have really been asked to flush some
framebuffer state otherwise we may do redundant work if multiple
framebuffers are pushed/popped before something is really drawn.
This integrates the face winding state tracking with the design we have
for handling most of the other framebuffer state so we benefit from the
optimizations for minimizing the cost of _cogl_framebuffer_flush_state()
Reviewed-by: Neil Roberts <neil@linux.intel.com>
We should not be deciding whether we need to really update the GL color
mask state at the point where a new framebuffer has been pushed, we
should be waiting until we have really been asked to flush some
framebuffer state otherwise we may do redundant work if multiple
framebuffers are pushed/popped before something is really drawn.
This integrates the color mask state tracking with the design we have
for handling most of the other framebuffer state so we benefit from the
optimizations for minimizing the cost of _cogl_framebuffer_flush_state()
Reviewed-by: Neil Roberts <neil@linux.intel.com>
Previously the cost of _cogl_framebuffer_state_flush() would always
scale by the total amount of state tracked by CoglFramebuffer even in
cases where we knew up-front that we only wanted to flush a subset of
the state or in cases where we requested to flush the same framebuffer
multiple times with no changes being made to the framebuffer.
We now track a set of state changed flags with each framebuffer and
track the current read/draw buffers as part of the CoglContext so that
we can quickly bail out when asked to flush the same framebuffer
multiple times with no changes.
_cogl_framebuffer_flush_state() now takes a mask of the state that we
want to flush and the implementation has been redesigned so that the
cost of checking what needs to be flushed and flushing those changes
now scales by how much state we actually plan to update.
Reviewed-by: Neil Roberts <neil@linux.intel.com>
There was only one place where we called _cogl_clip_state_flush() in
_cogl_framebuffer_flush_state() and we can just as well use
_cogl_clip_state_get_stack() and _cogl_clip_stack_flush() directly
instead.
Reviewed-by: Neil Roberts <neil@linux.intel.com>
The only place we were calling _cogl_clip_stack_dirty() was when
changing the current draw_buffer which also implies a change in
the current clip stack. _cogl_clip_stack_flush() would already
be able to quickly determine that the clip stack has changed by
checking ctx->current_clip_stack so there isn't really any need
to explicitly mark the clip_stack state as dirty.
Reviewed-by: Neil Roberts <neil@linux.intel.com>
The aim is to make cogl-framebuffer.c responsible for avoiding redundant
flushing of its matrix stacks so this removes the checks done directly
within cogl-matrix-stack.c.
Reviewed-by: Neil Roberts <neil@linux.intel.com>
This adds CoglFramebuffer methods for accessing the clip stack. We plan
on making some optimizations to how framebuffer state is flushed which
will require us to track when a framebuffer's clip state has changed.
This api also ties in to the longer term goal of removing the need for a
default global CoglContext since these methods are all implicitly
related to a specific context via their framebuffer argument.
Reviewed-by: Neil Roberts <neil@linux.intel.com>
This ensures we don't touch a framebuffer's matrix stack directly if we
are also relying on _cogl_framebuffer_flush_state(). We want to get to
the point where we can set dirty flags against framebuffer state at the
point it changes but that means we can't allow direct access to the
matrix stack. _cogl_texture_draw_and_read() has now been changed so it
uses cogl_framebuffer_ methods to update the matrix stacks including
adding new internal _cogl_framebuffer_push/pop_projection() functions
that allow us to set transient projections.
Reviewed-by: Neil Roberts <neil@linux.intel.com>
This adds a cogl_framebuffer_identity_matrix() method that can be used
to reset the current modelview matrix to the identity matrix.
Reviewed-by: Neil Roberts <neil@linux.intel.com>
This adds cogl_framebuffer_ methods to update the modelview and
projection matrix stacks to replace functions like cogl_translate(),
cogl_rotate() and cogl_scale() etc.
This is part of the on-going effort to get rid of the global CoglContext
pointer since the existing methods don't take an explicit pointer to a
CoglContext. All the methods are now related to a context via the
framebuffer.
We added framebuffer methods instead of direct context methods because
the matrix stacks are per-framebuffer and as well as removing the global
CoglContext we would rather aim for a more direct state access API
design than, say, cairo or OpenGL, so we'd like to avoid needing the
cogl_push/pop_framebuffer(). We anticipate that Cogl will mostly be
consumed by middleware graphics layers such as toolkits or game engines
and feel that a more stateless model will avoid impedance mismatches if
higher levels want to expose a stateless model to their developers and
statefullness can still be added by higher levels if really desired.
Reviewed-by: Neil Roberts <neil@linux.intel.com>
Cogl keeps a pointer to the last used onscreen framebuffer from the
context to implement the deprecated cogl_set_draw_buffer function
which can take COGL_WINDOW_BUFFER as the target to use the last
onscreen buffer. Previously this would also take a reference to that
pointer. However that was causing a circular reference between the
framebuffer and the context which makes it impossible to clean up
resources properly when onscreen buffers are used. This patch instead
changes it to just store the pointer and then clear the pointer during
_cogl_onscreen_free as a kind of cheap weak reference.
Reviewed-by: Robert Bragg <robert@linux.intel.com>
This adds two new experimental public functions to replace the old
internal _cogl_pipeline_set_cull_face_state function:
void
cogl_pipeline_set_cull_face_mode (CoglPipeline *pipeline,
CoglPipelineCullFaceMode cull_face_mode);
void
cogl_pipeline_set_front_face_winding (CoglPipeline *pipeline,
CoglWinding front_winding);
There are also the corresponding getters.
https://bugzilla.gnome.org/show_bug.cgi?id=663628
Reviewed-by: Robert Bragg <robert@linux.intel.com>
It's useful to be able to query back the number of
point_samples_per_pixel that may have previously be chosen using
cogl_framebuffer_set_samples_per_pixel().
Reviewed-by: Neil Roberts <neil@linux.intel.com>
Since we've had several developers from admirable projects say they
would like to use Cogl but would really prefer not to pull in
gobject,gmodule and glib as extra dependencies we are investigating if
we can get to the point where glib is only an optional dependency.
Actually we feel like we only make minimal use of glib anyway, so it may
well be quite straightforward to achieve this.
This adds a --disable-glib configure option that can be used to disable
features that depend on glib.
Actually --disable-glib doesn't strictly disable glib at this point
because it's more helpful if cogl continues to build as we make
incremental progress towards this.
The first use of glib that this patch tackles is the use of
g_return_val_if_fail and g_return_if_fail which have been replaced with
equivalent _COGL_RETURN_VAL_IF_FAIL and _COGL_RETURN_IF_FAIL macros.
Reviewed-by: Neil Roberts <neil@linux.intel.com>
This factors out the CoglOnscreen code from cogl-framebuffer.c so we now
have cogl-onscreen.c, cogl-onscreen.h and cogl-onscreen-private.h.
Notably some of the functions pulled out are currently namespaced as
cogl_framebuffer but we know we are planning on renaming them to be in
the cogl_onscreen namespace; such as cogl_framebuffer_swap_buffers().
Reviewed-by: Neil Roberts <neil@linux.intel.com>
Currently features are represented as bits in a 32bit mask so we
obviously can't have more than 32 features with that approach. The new
approach is to use the COGL_FLAGS_ macros which lets us handle bitmasks
without a size limit and we change the public api to accept individual
feature enums instead of a mask. This way there is no limit on the
number of features we can add to Cogl.
Instead of using cogl_features_available() there is a new
cogl_has_feature() function and for checking multiple features there is
cogl_has_features() which takes a zero terminated vararg list of
features.
In addition to being able to check for individual features this also
adds a way to query all the features currently available via
cogl_foreach_feature() which will call a callback for each feature.
Since the new functions take an explicit context pointer there is also
no longer any ambiguity over when users can first start to query
features.
Reviewed-by: Neil Roberts <neil@linux.intel.com>
Cogl doesn't expose public api for blitting between framebuffers so it
doesn't make much sense to have this feature as part of the public api
currently. We can't break the api by removing the enum but at least we
no longer ever set the feature flag.
We now have a replacement private feature flag
COGL_PRIVATE_FEATURE_OFFSCREEN_BLIT which cogl now checks for
internally.
Reviewed-by: Neil Roberts <neil@linux.intel.com>
This adds a new experimental function, cogl_framebuffer_finish(), which
can be used to explicitly synchronize the CPU with the GPU. It's rare
that this level of explicit synchronization is desirable but for example
it can be useful during performance analysys to make sure measurements
reflect the working time of the GPU not just the time to queue commands.
Reviewed-by: Neil Roberts <neil@linux.intel.com>
This adds support for multisample rendering to offscreen framebuffers.
After an offscreen framebuffer is first instantiated using
cogl_offscreen_new_to_texture() it is then possible to use
cogl_framebuffer_set_samples_per_pixel() to request multisampling before
the framebuffer is allocated. This also adds
cogl_framebuffer_resolve_samples() for explicitly resolving point
samples into pixels. Even though we currently only support the
IMG_multisampled_render_to_texture extension which doesn't require an
explicit resolve, the plan is to also support the
EXT_framebuffer_multisample extension which uses the framebuffer_blit
extension to issue an explicit resolve.
Reviewed-by: Neil Roberts <neil@linux.intel.com>
When creating new onscreen framebuffers we need to take the
configuration in cogl terms and translate that into a configuration
applicable to any given winsys, e.g. an EGLConfig or a GLXFBConfig
or a PIXELFORMATDESCRIPTOR.
Also when we first create a context we typically have to do a very
similar thing because most OpenGL winsys APIs also associate a
framebuffer config with the context and all future configs need to be
compatible with that.
This patch introduces an internal CoglFramebufferConfig to wrap up some
of the configuration parameters that are common to CoglOnscreenTemplate
and to CoglFramebuffer so we aim to re-use code when dealing with the
above two problems.
This patch also aims to rework the winsys code so it can be more
naturally extended as we start adding more configureability to how
onscreen framebuffers are created.
Reviewed-by: Neil Roberts <neil@linux.intel.com>
This adds cogl_framebuffer_discard_buffers API that allows applications
to explicitly discard depth and stencil buffers which really helps when
using a tile based GPU architexture by potentially avoiding the need to
save the results of depth and stencil buffer changes to system memory
between frames since these can usually be handled directly with on-chip
memory instead.
The semantics for cogl_framebuffer_swap_buffers and
cogl_framebuffer_swap_region are now documented to include an implicit
discard of all buffers, including the color buffer.
We now recommend that all rendering to a CoglOffscreen framebuffer
should be followed by a call like:
cogl_framebuffer_discard_buffers (fb,
COGL_BUFFER_BIT_DEPTH|
COGL_BUFFER_BIT_STENCIL);
Reviewed-by: Neil Roberts <neil@linux.intel.com>
As part of the on going, incremental effort to purge the non type safe
CoglHandle type from the Cogl API this patch tackles most of the
CoglHandle uses relating to textures.
We'd postponed making this change for quite a while because we wanted to
have a clearer understanding of how we wanted to evolve the texture APIs
towards Cogl 2.0 before exposing type safety here which would be
difficult to change later since it would imply breaking APIs.
The basic idea that we are steering towards now is that CoglTexture
can be considered to be the most primitive interface we have for any
object representing a texture. The texture interface would provide
roughly these methods:
cogl_texture_get_width
cogl_texture_get_height
cogl_texture_can_repeat
cogl_texture_can_mipmap
cogl_texture_generate_mipmap;
cogl_texture_get_format
cogl_texture_set_region
cogl_texture_get_region
Besides the texture interface we will then start to expose types
corresponding to specific texture types: CoglTexture2D,
CoglTexture3D, CoglTexture2DSliced, CoglSubTexture, CoglAtlasTexture and
CoglTexturePixmapX11.
We will then also expose an interface for the high-level texture types
we have (such as CoglTexture2DSlice, CoglSubTexture and
CoglAtlasTexture) called CoglMetaTexture. CoglMetaTexture is an
additional interface that lets you iterate a virtual region of a meta
texture and get mappings of primitive textures to sub-regions of that
virtual region. Internally we already have this kind of abstraction for
dealing with sliced texture, sub-textures and atlas textures in a
consistent way, so this will just make that abstraction public. The aim
here is to clarify that there is a difference between primitive textures
(CoglTexture2D/3D) and some of the other high-level textures, and also
enable developers to implement primitives that can support meta textures
since they can only be used with the cogl_rectangle API currently.
The thing that's not so clean-cut with this are the texture constructors
we have currently; such as cogl_texture_new_from_file which no longer
make sense when CoglTexture is considered to be an interface. These
will basically just become convenient factory functions and it's just a
bit unusual that they are within the cogl_texture namespace. It's worth
noting here that all the texture type APIs will also have their own type
specific constructors so these functions will only be used for the
convenience of being able to create a texture without really wanting to
know the details of what type of texture you need. Longer term for 2.0
we may come up with replacement names for these factory functions or the
other thing we are considering is designing some asynchronous factory
functions instead since it's so often detrimental to application
performance to be blocked waiting for a texture to be uploaded to the
GPU.
Reviewed-by: Neil Roberts <neil@linux.intel.com>
This adds an internal function to set the backface culling state on a
pipeline. This includes properties to set the culling mode (front,
back or both) and also to set which face is considered the front
(COGL_WINDING_CLOCKWISE or COGL_WINDING_COUNTER_CLOCKWISE). The actual
front face flushed to GL depends on whether we are rendering to an
offscreen buffer or not. This means that when changing between on- and
off- screen framebuffers it now checks whether the last flushed
pipeline has backface culling enabled and forces a reflush of the cull
face state if so.
The backface culling is now set on a pipeline as part of the legacy
state. This is important because some code in Cogl assumes it can
flush a temporary pipeline to revert to a known state, but previously
this wouldn't disable backface culling so things such as flushing the
clip stack could get confused.
Reviewed-by: Robert Bragg <robert@linux.intel.com>
When changing between two framebuffers that have different color masks
it now forces the pipeline to flush the mask by setting
current_pipeline_changes_since_flush. For this to work there needs to
be a common bit of code that gets called when the framebuffers are
changed that has access to both the old framebuffer and the new
framebuffer. _cogl_set_framebuffers_real can't be used for this
because when it is called from cogl_pop_framebuffer the stack entries
have already changed so it can't know the old framebuffer. This patch
adds a new function called notify_buffers_changed which should get
called whenever the buffers are changed and it explicitly gets passed
pointers to the old and new buffers. cogl_pop_framebuffer now calls
this instead of trying to use _cogl_set_framebuffers_real to force a
flush.
This patch also fixes the ctx->window_buffer pointer. Previously this
was implemented by searching in the framebuffer stack for an onscreen
framebuffer whenever the current buffers are changed. However it does
this after the stack has already changed so it won't usually find the
right buffer.
Reviewed-by: Robert Bragg <robert@linux.intel.com>
This adds a new function, cogl_framebuffer_get_color_format() to be able
to query the common pixel format for any color buffers attached to a
given CoglFramebuffer. For example an offscreen framebuffer created
using cogl_offscreen_new_to_texture() would have a format matching the
texture.
Reviewed-by: Neil Roberts <neil@linux.intel.com>
Cogl aims to consistently put the origin of 2D objects at the top-left
instead of the bottom left as OpenGL does, but there was an oversight
and the experimental cogl_framebuffer_swap_region API was accepting
coordinates relative to the bottom left. Cogl will now flip the user's
given rectangles to be relative to the bottom of the framebufffer before
sending them to APIs like glXCopySubBuffer and glBlitFramebuffer.
Reviewed-by: Neil Roberts <neil@linux.intel.com>
If the user doesn't explicitly allocate a CoglFramebuffer then Cogl
should automatically allocate the framebuffer when the user starts to
draw to the framebuffer. So this way calling cogl_framebuffer_allocate
is only required if you are explicitly interested in checking for and
gracefully handling failures to allocate a framebuffer. If automatic
allocation fails then application behaviour becomes undefined.
Reviewed-by: Neil Roberts <neil@linux.intel.com>
This makes cogl_framebuffer_clear and cogl_framebuffer_clear4f public as
experimental API. Since these functions take explicit framebuffer
pointers you don't need to push/pop a framebuffer just to clear it. Also
these functions are implicitly tied to a specific CoglContext via the
framebuffer pointer unlike cogl_clear.
Reviewed-by: Neil Roberts <neil@linux.intel.com>
Some of the functions we were calling in cogl_framebuffer_clear[4f] were
referring to the current framebuffer, which would result in a crash
if nothing had been pushed before trying to explicitly clear a given
framebuffer.
Reviewed-by: Neil Roberts <neil@linux.intel.com>
There is no need to call _cogl_framebuffer_init_bits for the draw and
read buffers each time we flush the framebuffer state since we will
always re-sync with gl if necessary when the
cogl_framebuffer_get_red/green/blue/alpha_bits functions are called.
Reviewed-by: Neil Roberts <neil@linux.intel.com>
This adds a function to query what CoglContext a given framebuffer
belongs too. This can be useful if you pass framebuffer pointers around
and at some point you want to create another framebuffer as part of the
same context as a given framebuffer without assuming there is a single
default context.
Reviewed-by: Neil Roberts <neil@linux.intel.com>
When we need to guarantee that the glColorMask is re-asserted the next
time that a primitive is drawn it is not enough to just OR in the
LOGIC_OPS flag to ctx->current_pipeline_changes_since_flush because
_cogl_pipeline_flush_gl_state actually checks the age of the pipeline
before checking that. If the pipeline hasn't aged then we bail out
early. This makes sure we decrement
ctx->current_pipeline_changes_since_flush so the next time we come to
flush a pipeline we will see a differing age.
Reviewed-by: Neil Roberts <neil@linux.intel.com>
This adds CoglPipeline and CoglFramebuffer support for setting a color
mask which is a bit mask defining which color channels should be written
to the current framebuffer.
The final color mask is the intersection of the framebuffer color mask
and the pipeline color mask. The framebuffer mask affects all rendering
to the framebuffer while the pipeline masks can be used to affect
individual primitives.
Reviewed-by: Neil Roberts <neil@linux.intel.com>
This adds a getter and setter for requesting dithering to be enabled.
Dithering is a hardware dependent technique to increase the visible
color resolution beyond what the underlying hardware supports by playing
tricks with the colors placed into the framebuffer to give the illusion
of other colors. (For example this can be compared to half-toning used
by some news papers to show varying levels of grey even though their may
only be black and white are available).
The results of enabling dithering are platform dependent and may have no
effect.
Signed-off-by: Neil Roberts <neil@linux.intel.com>
There were several CoglOnscreen functions named like:
cogl_onscreen_<platform>_blah instead of cogl_<platform>_onscreen_blah
so this patch updates those to be consistent with other platform
specific apis we have in cogl.
Signed-off-by: Neil Roberts <neil@linux.intel.com>
The GL or GLES library is now dynamically loaded by the CoglRenderer
so that it can choose between GL, GLES1 and GLES2 at runtime. The
library is loaded by the renderer because it needs to be done before
calling eglInitialize. There is a new environment variable called
COGL_DRIVER to choose between gl, gles1 or gles2.
The #ifdefs for HAVE_COGL_GL, HAVE_COGL_GLES and HAVE_COGL_GLES2 have
been changed so that they don't assume the ifdefs are mutually
exclusive. They haven't been removed entirely so that it's possible to
compile the GLES backends without the the enums from the GL headers.
When using GLX the winsys additionally dynamically loads libGL because
that also contains the GLX API. It can't be linked in directly because
that would probably conflict with the GLES API if the EGL is
selected. When compiling with EGL support the library links directly
to libEGL because it doesn't contain any GL API so it shouldn't have
any conflicts.
When building for WGL or OSX Cogl still directly links against the GL
API so there is a #define in config.h so that Cogl won't try to dlopen
the library.
Cogl-pango previously had a #ifdef to detect when the GL backend is
used so that it can sneakily pass GL_QUADS to
cogl_vertex_buffer_draw. This is now changed so that it queries the
CoglContext for the backend. However to get this to work Cogl now
needs to export the _cogl_context_get_default symbol and cogl-pango
needs some extra -I flags to so that it can include
cogl-context-private.h
cogl-ext-functions.h now contains definitions for all of the core GL
and GLES functions that we would normally link to directly. All of the
code has changed to access them through the cogl context pointer. The
GE macro now takes an extra parameter to specify the context because
the macro itself needs to make GL calls but various points in the Cogl
source use different names for the context variable.
Instead of storing all of the feature function pointers in the driver
specific data of the CoglContext they are now all stored directly in
CoglContext. There is a single header containing the description of
the functions which gets included by cogl-context.h. There is a single
function in cogl-feature-private.c to check for all of these
functions.
The name of the function pointer variables have been changed from
ctx->drv.pf_glWhatever to just ctx->glWhatever.
The feature flags that get set when an extension is available are now
separated from the table of extensions. This is necessary because
different extensions can mean different things on GLES and GL. For
example, having access to glMapBuffer implies read and write support
on GL but only write support on GLES. The flags are instead set in the
driver specific init function by checking whether the function
pointers were successfully resolved.
_cogl_feature_check has been changed to assume the feature is
supported if any of the listed extensions are available instead of
requiring all of them. This makes it more convenient to specify
alternate names for the extension. Nothing else had previously listed
more than one name for an extension so this shouldn't cause any
problems.
This exposes the previously internal only
_cogl_framebuffer_get_red/green/blue/alpha_bits() functions as 2.0
experimental API.
Signed-off-by: Neil Roberts <neil@linux.intel.com>
This exposes experimental cogl_framebuffer APIs for getting and setting
a viewport without having to refer to the implicit CoglContext. It adds
the following experimental API:
cogl_framebuffer_set_viewport
cogl_framebuffer_get_viewport4fv
cogl_framebuffer_get_viewport_x
cogl_framebuffer_get_viewport_y
cogl_framebuffer_get_viewport_width
cogl_framebuffer_get_viewport_height
Signed-off-by: Neil Roberts <neil@linux.intel.com>
Instead of the stub winsys being a special case set of #ifdef'd code
used when COGL_HAS_FULL_WINSYS wasn't defined, the stub winsys now
implements a CoglWinsysVtable like all other winsys backends (it's just
that everything is a NOP). This way we can get rid of the
COGL_HAS_FULL_WINSYS define and also the stub winsys can be runtime
selected whereas before it was incompatible with all other winsys
backends.
This validates that the viewport width and height arguments are positive
values in _cogl_framebuffer_set_viewport. In addition, just before
calling glViewport we also assert that something else hasn't gone amiss
and that the internal viewport width/height values we track are still
positive before passing to glViewport which generates an error for
negative values.
With GLES 1, frame buffers are a optional extensions. We need to make
sure the pointer exist before calling the function and do that by just
checkout the corresponding feature.
When freeing a framebuffer stack it's possible to have entries with NULL
draw or read buffers so we should check that before calling
cogl_onscreen/offscreen_free. This fixes a crash with the wayland
backend when running conformance tests such as cogl-test-object which
never push a framebuffer.
