We're going to enforce some invariants a bit stricter and will only
allow allocating if an actor is mapped, not only visible.
Since actors are only mapped if their parent is mapped and stages are
hidden by default, we need to show the stage to ensure the actors are
mapped before we allocate them. So do that and call clutter_actor_show()
on the stage before fake-allocating the test actors.
https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1366
650efb6445