As per commit 044572cb boxpointer uses its own coordinates to position itself.
However this would lead to warning when mutter-clutter is compiled with debug
options as we'd might try to set the box coordinates during the allocation
cycle.

So, when calling _reposition during allocation, instead of setting the actor's
coordinates we just pass the allocation box and we adjust its origin, in order
to set it properly in the vfunc.

Fixes GNOME/gnome-shell#1382
GNOME/gnome-shell!576