When profiling gnome-shell it was found that one of the main triggers
of `clutter_actor_queue_relayout` during animations was
`clutter_actor_set_margin_internal` continuously setting the same
zero margins. That's obviously pointless and also expensive. So just
avoid redundant margin changes.
This helps to further improve performance in:
https://gitlab.gnome.org/GNOME/mutter/issues/233,
https://gitlab.gnome.org/GNOME/gnome-shell/issues/349
This change previously landed as 59acb3895 and then got reverted because
it was found to make gnome-shell#517 worse. However that bug now has a
proper fix and this branch isn't really directly related so is being
reproposed...
e0fd7a6d05