MetaCompositorView is a class which contains compositor logic
specific to ClutterStageViews.
Each MetaCompositorView is "attached" to a ClutterStageView as an
opaque pointer using g_object_set_qdata_full (), and is freed when
the ClutterStageView is destroyed. This ensures that the lifetime of
the MetaCompositorView can't extend beyond the lifetime of its
ClutterStageView.
In a following commit, MetaCompositorView will be expanded to allow
keeping track of the top MetaWindowActor located on each
ClutterStageView.
Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/2526>
