gl-renderer: add support for (a|x)bgr2101010 shm formats

Adding these formats makes it possible for clients using wl_shm to
submit buffers with 10 bits per pixel, and thus (if Weston is
configured with an xrgb2101010 frame buffer) display more precise
colors on some computer monitors.

Signed-off-by: Manuel Stoeckl <code@mstoeckl.com>