Avoid popping the workspace controls in and out at the end of DND

Avoid popping the workspace controls in and out at the end of DND

At the end of a drag operation, we would invoke the code to slide the
controls in (because we were no longer DND'ing and not hovering) and
then immediately afterwards invoke the code to slide it back out when
we got the ENTER event from the end of DND. While the immediately
overridden tween probably won't have any visible effect it's better
to avoid this, so wait to update the zoom state until BEFORE_REDRAW.

https://bugzilla.gnome.org/show_bug.cgi?id=640996