First, add logic in MetaCompositorView to find topmost visible
MetaWindowActor on its view, and expose it through a new API.
Then, queue an update to find the top MetaWindowActor of each
MetaCompositorView in the following cases:
1. The MetaCompositor is in its initial state.
2. The window stack order has changed.
3. A window has changed its visibility.
4. A "stage-views-changed" signal was emitted for a MetaWindowActor.
Finally, perform the queued update in meta_compositor_before_paint (),
and assert that an update isn't queued during painting. This ensures
that the top window actor in the MetaCompositorView remains up-to-date
and available to child classes of MetaCompositor throughout the entire
paint stage.
Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/2526>
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>
