gl-renderer: add support for (a|x)bgr16161616 shm formats

These formats are useful because they are often easier to produce
on CPU than half-float formats, and abgr16161616 has both >= 10bpc
color channels and adequate alpha, unlike abgr2101010.

The 16-bpc textures created from buffers with these formats require
the GL_EXT_texture_norm16 extension.

As WL_SHM_FORMAT_ABGR16161616 was introduced in libwayland 1.20,
update Weston's build requirements and CI.

The formats also needed to be registered in the pixel format table,
and defined in a fallback path if recent libdrm is not available.

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