Add support for [ax]bgr16161616 wl_shm formats

The ABGR16161616 format in particular is useful because it lets clients submit buffers which have both >= 10bpc color and a nontrivial alpha channel, unlike ABGR2101010 or ABGR88888 Wayland pixel formats.

Note: this increases the required Wayland version to 1.20, which was released less than 2 months ago. It looks like the FreeBSD CI is failing because of this.