Track the stereo status of windows using the new EXT_stereo_tree
GLX extension.
When stereo is enabled or disabled, a restart is triggered via
meta_restart() after a timeout, setting a _META_ENABLE_STEREO
property on the root window to indicate whether we should
turn on a stereo stage for clutter. The property avoids a loop,
since we need to enable stereo *before* initializing Clutter and GL,
but we need GL to figure out whether we have stereo windows.
Stereo windows are drawn to the stage using new functionality
in Cogl to setup a stereo context, select which buffer to draw
to, and draw either the left or right buffer of a stereo
texture_from_pixmap.
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