If we want to show quad-buffer stereo with Cogl, we need to pick an
appropriate fbconfig for creating the CoglOnscreen objects. Add
cogl_onscreen_template_set_stereo_enabled() to indicate whether
stereo support is needed.
Add cogl_framebuffer_get_stereo_mode() to see if a framebuffer was
created with stereo support.
Add cogl_framebuffer_get_stereo_mode() to pick whether to draw to
the left, right, or both buffers.
Reviewed-by: Robert Bragg <robert.bragg@intel.com>
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