The default stage was a neat concept when we started Clutter out,
somewhere in the Jurassic era; a singleton instance that gets created at
initialization time, and remains the same for the entire duration of the
process.
Worked well enough when Clutter was a small library meant to be used to
write fullscreen media browsers, but since the introduction of multiple
stages, and Clutter being used to create all sorts of applications, the
default stage is just a vestigial remainder of that past, like an
appendix; something that complicates the layout of the code and
introduces weird behaviour, so that you notice its existence only when
something goes wrong.
Some platforms we do support, though, only have one framebuffer, so it
makes sense for them to have only one stage.
At this point, the only sane thing to do is to go through the same code
paths on all platforms, and that code path is the stage instance
creation and initialization — i.e. clutter_stage_new() (or
g_object_new() with CLUTTER_TYPE_STAGE).
For platforms that support multiple stages, nothing has changed: the stage
created by clutter_stage_get_default() will be set as the default one;
if nobody calls it, the default stage is never created, and it just
lives on as a meaningless check.
For platforms that only support one stage, clutter_stage_new() and
clutter_stage_get_default() will behave exactly the same the first time
they are called: both will create a stage, and set it as the default.
Calling clutter_stage_new() a second time is treated as a programmer
error, and will result in Clutter aborting. This is a behavioural change
because the existing behaviour or creating a new ClutterStage instance
with the same ClutterStageWindow private implementation is, simply put,
utterly braindamaged and I should have *never* had written it, and I
apologize for it. In my defence, I didn't know any better at the time.
This is the first step towards the complete deprecation of
clutter_stage_get_default() and clutter_stage_is_default(), which will
come later.
