clutter: Avoid redundant margin changes [performance]
When profiling gnome-shell it was found that one of the main triggers
clutter_actor_queue_relayout during animations was
clutter_actor_set_margin_internal continuously setting the same
zero margins. That's obviously pointless but also expensive since it
incurs full stage relayouts and reallocation. So just avoid redundant
FIXME: This merged then got reverted. Needs a new MR.