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.

The new formats can be tested with wlfmts (https://git.sr.ht/~mstoeckl/wlfmts) or gradient-test (from https://gitlab.freedesktop.org/jadahl/wayland-test-clients), or (setting LIBGL_ALWAYS_SOFTWARE=1) an OpenGL program like imv.