This introduces the option --drm-format (currently used only by
context_drm_egl, vo_drm implementation is pending) which allows you to
pick between a xrgb8888 or a xrgb2101010 visual for --gpu-context=drm.
Requires a recent mesa (18.0.0_rc4 or later) to work.
This also fixes a bug when using --gpu-context=drm on a 30bpp-enabled
mesa (allow_rgb10_configs set to true). Previously it would've set up
an XRGB8888 format at the DRM/GBM level, while a 30bpp EGLConfig would
be picked, resulting in a garbled image.
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