backend-x11: use DRM formats for EGLConfig

backend-x11: use DRM formats for EGLConfig

Define a specific DRM format for the GL-renderer to render in. It goes through
fuzzy matching in egl-glue.c which ensures we get exactly the number of bits
for each channel, but does not require an exact format match.

This ensures we get the bit depth we expect instead of the first arbitrary
EGLConfig.

This should not change the current behaviour, because Mesa EGL takes care to
order the configs as apps expect.

Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.com>