Subsurfaces can be effectively synced indirectly via their ancestors.
Right now such indirectly synced surfaces don't apply their cached
state when their ancestor effectively becomes desync as by the time
we call `parent_state_applied()` on them, they are considered as
desync.
Thus sligthly reoder things so when the ancestors becomes desync
and applies its state, those surfaces still count as synced and
will thus apply their cached state as well.
While on it, add a check to prevent `set_desync()` to have side
effects when the target surface is not currently synced.
Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/2232>
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