pixman-renderer: change repaint_region() arguments

Change the region argument types in repaint_region(), moving the
final_region computation to the caller. The caller is in a better
position deciding if source clipping is needed or if it can be intersected
into the final_region via a simple translation. This avoids
surf_region or source clip implying that the transformation is only a
translation.

The region_global_to_output() call is also moved into the callers so
that repaint_region() would not modify caller-provided data. Modifying
caller provided data could be surprising.

This patch does not change the rendering output.

v2: Remove unused source_clip argument.

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>