The MetaWaylandPointer used to put this together through
MetaCursorTracker cursor visibility, and ClutterSeat-level
inhibition API, applying the pointer focus changes due to
visibility logically to Wayland clients.
In order to make this work over all Clutter widgetry
instead of just Wayland clients, make the ClutterSeat-level
inhibition API control this feature at the ClutterStage picking
level, and leave/enter the seat pointer as appropriate.
By default, the seat pointer has (un)focus inhibited. The
MetaCursorTracker has been made another player in unfocus
inhibition, simply asking for the pointer to get its focus
while the cursor is visible.
This in practice means that picking code may return a NULL
actor, some asserts and preconditions had to be changed to
handle this, plus some test code slightly.
Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/3420>
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