We would get the MetaDisplay from the backend singleton before creating
the MetaCompositor, then in MetaCompositor, get the backend singleton
again to get the stage. To get rid of the extra singleton fetching, just
pass the backend the MetaCompositor constructors, and fetch the stage
directly from the backend everytime it's needed.
This also makes it available earlier than before, as we didn't set our
instance private stage pointer until the manage() call.
https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1289
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
