pixman-renderer: simplify the output-to-buffer matrix computation

pixman-renderer: simplify the output-to-buffer matrix computation

Now that we have a buffer-to-surface matrix and the global-to-output matrix
is in pixels, we can remove a large chunk of confusing code from the pixman
renderer.  Hopefully, having this stuff in weston core will keep the pixman
renderer from gettin broken quite as often.

This patch makes attempting zoom on the pixman-renderer render funny
stuff. We didn't support zoom before, now it renders wrong instead of
not zooming at all.

[Pekka: adjust commit message]
Tested-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>
Reviewed-By: Derek Foreman <derekf@osg.samsung.com>
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>