This avoids having to hardcode the same fallbacks elsewhere multiple
times when determining what formats might be suitable for a set of
CRTCs. The formats_modifiers hash table is now guaranteed to be
populated with at least something, so future code will not need to
handle it being empty.
The hardcoded fallback formats are a minimal set probably supported by
most hardware. XRGB8888 is the format that, according to ancient lore,
all DRM devices should support, especially if they don't have the
capability to advertise otherwise. Mutter also hardcodes XRGB8888 as the
GBM surface format, so it is already required on primary GPUs.
XBGR8888 matches the most common OpenGL format, sans alpha channel since
scanout hardware has not traditionally supported alpha. XBGR8888 is here
also because Mutter hardcodes that format for secondary GPU outputs when
using the CPU copy path.
https://gitlab.gnome.org/GNOME/mutter/merge_requests/341
8a0d0ce987