boxpointer: Don't trigger a relayout when updating arrow side

boxpointer: Don't trigger a relayout when updating arrow side

Changing the arrow side might need to reposition the boxpointer, however
if this happens during allocation, we don't need to trigger a new relayout since
we'd set the new allocation once _updateFlip's _reposition call is terminated,
otherwise if the position has changed, changing the boxpointer coordinates
will trigger a relayout anyways.

https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/576