A base type shouldn't know about sub types, so let MetaDisplay make
the correct choice of what type of MetaCompositor it should create. No
other semantical changes introduced.
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