compositor: Track which plane a surface is on

We start tracking which hardware plane a surface is displayed on, which
lets us avoid generating damage when a hardware overlay/cursor is moved
around.
diff --git a/src/compositor.c b/src/compositor.c
index c93da34..73fed3e 100644
--- a/src/compositor.c
+++ b/src/compositor.c
@@ -306,6 +306,9 @@
 	struct weston_compositor *compositor = surface->compositor;
 	pixman_region32_t damage;
 
+	if (surface->plane != WESTON_PLANE_PRIMARY)
+		return;
+
 	pixman_region32_init(&damage);
 	pixman_region32_subtract(&damage, &surface->transform.boundingbox,
 				 &surface->clip);