It doesn't use anything specific to MetaCompositor, and MetaCompositorX11 isn't exposed, so move it to MetaX11Display. https://gitlab.gnome.org/GNOME/mutter/merge_requests/727
Introduce MetaCompositorX11, dealing with being a X11 compositor, and MetaCompositorServer, being a compositor while also being the display server itself, e.g. a Wayland display server. https://gitlab.gnome.org/GNOME/mutter/merge_requests/727