compositor: add weston_surface_to_buffer_region()

This will be used by pixman-renderer.

v2: Fix doc typo.

Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>
Reviewed-By: Derek Foreman <derekf@osg.samsung.com>
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>