If an actor sets flag `CLUTTER_ACTOR_NO_LAYOUT` then that means it
is (or should be) unaffected by `queue_relayout` calls in its children.
So we can avoid propagating `queue_relayout` all the way up to the stage
and avoid a full stage relayout each time.
But those children whose parent has `CLUTTER_ACTOR_NO_LAYOUT` still need
to be allocated at some point. So we do it at the same point where it
happened before. Only we now queue a *shallow* relayout so the `allocate`
run on the next frame doesn't need to descend the whole actor tree anymore.
Only a subtree and hopefully very small.
For free-floating and top-level actors this provides a measurable
performance benefit. According to Google Profiler, calls to
`_clutter_stage_maybe_relayout` are now so cheap that they no longer show
up in performance profiles.
https://gitlab.gnome.org/GNOME/mutter/merge_requests/575
2731be6929