mesa: Add R8G8_R8B8 and G8R8_B8R8 formats

mesa: Add R8G8_R8B8 and G8R8_B8R8 formats

These aren't real formats.  They cannot be used with
RenderbufferObjects, Textures, or anything else visible through the GL
API.  The only purpose is to support YUYV textures imported through
EGL_image_external.  There is hardware that can sample from this
subsampled format, but the hardware does not do the colorspace
conversion automatically.

v2: Treat these formats as compressed.  This is a lie, but it seems to
be a less egregious lie than what I was doing before.  It also passes
'ninja test'.

Reviewed-by: Emma Anholt <emma@anholt.net>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/9610>