compositor: Add damage_buffer

Add an implementation of wl_surface.damage_buffer, similar to
wl_surface.damage except it uses buffer co-ordinates.

Reviewed-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>
Signed-off-by: Derek Foreman <derekf@osg.samsung.com>
diff --git a/src/compositor.h b/src/compositor.h
index 1d04481..43db92e 100644
--- a/src/compositor.h
+++ b/src/compositor.h
@@ -931,7 +931,9 @@
 	int32_t sy;
 
 	/* wl_surface.damage */
-	pixman_region32_t damage;
+	pixman_region32_t damage_surface;
+	/* wl_surface.damage_buffer */
+	pixman_region32_t damage_buffer;
 
 	/* wl_surface.set_opaque_region */
 	pixman_region32_t opaque;