compositor: Move weston_matrix_transform_region to compositor.c and export it

We're going to use this to replace much of the other transform code so
it's no longer just relevant to pixman-renderer.c

Signed-off-by: Derek Foreman <derekf@osg.samsung.com>
[Pekka: add the warning about matrix restrictions]
Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>
3 files changed