wayland: rework resizing (again)

Moves a good chunk of the resizing code to wayland_common.c. This makes it
possible to share it with future video drivers.

It doesn't resizit it immediatly, it calcutlates the new position and size and
then shedules a resizing event. This removes the ugly callback and void pointer
from the wayland data structure.
wayland: fix shm resizing for good