compositor: Create the inverse of the output matrix

Signed-off-by: Derek Foreman <derekf@osg.samsung.com>
Reviewed-by: Bryce Harrington <b.harrington@samsung.com>
Reviewed-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>
diff --git a/src/compositor.h b/src/compositor.h
index 24ed4fc..be32312 100644
--- a/src/compositor.h
+++ b/src/compositor.h
@@ -185,6 +185,8 @@
 
 	/** From global to output buffer coordinates. */
 	struct weston_matrix matrix;
+	/** From output buffer to global coordinates. */
+	struct weston_matrix inverse_matrix;
 
 	struct wl_list animation_list;
 	int32_t x, y, width, height;