Add support for 1555 and 4444 formats, both in RGB/BGR ordering, with
and without alpha.
These are already supported by Gallium and drivers, but not yet for
winsys surfaces. Adding these is enough to make them renderable when
using Weston on iris.
Signed-off-by: Daniel Stone <daniels@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/24331>