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>
diff --git a/src/compositor.h b/src/compositor.h
index ead03ae..76b0778 100644
--- a/src/compositor.h
+++ b/src/compositor.h
@@ -1034,6 +1034,11 @@
 			      pixman_box32_t rect);
 
 void
+weston_surface_to_buffer_region(struct weston_surface *surface,
+				pixman_region32_t *surface_region,
+				pixman_region32_t *buffer_region);
+
+void
 weston_spring_init(struct weston_spring *spring,
 		   double k, double current, double target);
 void