compositor: Add surface commit signal

Signed-off-by: Jonas Ådahl <jadahl@gmail.com>
Reviewed-by: Daniel Stone <daniels@collabora.com>
diff --git a/libweston/compositor.h b/libweston/compositor.h
index 4719338..9219567 100644
--- a/libweston/compositor.h
+++ b/libweston/compositor.h
@@ -1015,6 +1015,7 @@
 	struct wl_resource *resource;
 	struct wl_signal destroy_signal; /* callback argument: this surface */
 	struct weston_compositor *compositor;
+	struct wl_signal commit_signal;
 
 	/** Damage in local coordinates from the client, for tex upload. */
 	pixman_region32_t damage;