compositor-fbdev: fix output transformations

The pixman-renderer is already performing transformations when compositing
into the shadow buffer, we just need to get the damage co-ordinates right
when copying from shadow to front.

Signed-off-by: Derek Foreman <derekf@osg.samsung.com>
Reviewed-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>
Tested-by: Thilo Cestonaro <thilo@cestona.ro>