When a Wayland window acks our arrangement and we don't really have
anything to modify, we'll pass a sole flag of META_IS_WAYLAND_RESIZE
to meta_window_move_resize_internal using a garbage rect. The existing
code to calculate the new rectangle couldn't really handle this case,
and so the garbage rectangle accidentally got stored. Revamp the flag
checks to be more clear about it.

This fixes the weird positioning issues that sometimes appear when
resizing weston-terminal among others.