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 updates to CI were done by looking at earlier commits and following instructions in the comments; I may still have gotten something wrong. (Edit: like today's date, but I don't think that's a major problem.)