This way we know the stage extents without poking the stage. Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1403>
This object can be passed to random places, and be trusted to remain unchanged till replaced. Makes it an ideal replacement for MetaMonitorManager across threads. Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1403>