Since we often call meta_window_move_resize_now immediately after
mapping a window, we need to make sure that the placed coordinates
are saved in the unconstrained_rect. Ideally, placement positions
wouldn't be part of the constraints system, but instead are just
done inside meta_window_move_resize_internal as part of a special
path.

We're still working out the kinks of one large-scale refactor, so
it's best not to do another one while the first is going on. This
would be a great future cleanup, though: untangling constraints
and placement, alongside the force_placement state machine and
friends.