There is a race condition when we resize a stage before showing
it on X11.
The race goes like this:
- clutter_init() creates the default stage and realize it, which
will cause a 640x480 Window to be created
- call set_size(800, 600) on the stage will cause the Window to be
resized to 800x600
- call show() on the stage for the first time will cause COGL
to set up an 800 by 600 GL viewport
- the Window will be mapped, which will cause X to notify the
window manager that the Window should be resized to 800x600
- the window manager will approve the resize
- X resizes the drawable to 800x600
To fix the race, we need to defer COGL from setting up the viewport
until we receive a ConfigureNotify event and the X server has resized
the Drawable.
In order to defer the call to cogl_setup_viewport() we add a new
private flag, CLUTTER_STAGE_IN_RESIZE; the flag is checked whenever
we need to change the viewport size along with the SYNC_MATRICES
private flag. Thus, cogl_setup_viewport() will be called only if
SYNC_MATRICES is set and IN_RESIZE is not set.
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