This implements a mostly working native backend for Wayland. It can
be enabled via `gfx.webrender.compositor.force-enabled`.

The focus here was to get a basic structure in place while mini-
mising changes in shared code.

Known issues and limitations:
 - No readback - this will likely require an internal compositor
   again, as Wayland doesn't allow easily allow readback of the
   composited image, at least not without asking for permission.
   Alternatively, a new Wayland extension could be written for it.
 - Frame-call related issues when using a compositor that optimizes
   them (e.g. Gnome-Shell). This will be fixed in a follow-up, in
   the mean time disabling `widget.wayland.opaque-region.enabled`
   and `widget.wayland.vsync.enabled` works around the issues.
 - Only works on Weston or very recent versions of Gnome-Shell, see
   bug 1699754

Differential Revision: https://phabricator.services.mozilla.com/D111662