layer-shell: fix commit handler (issue #71)

On commit, do not arrange layers unless the layer shell-specific state
changed or the layer was (un)mapped.

Chase swaywm/sway@5fd5d643 and swaywm/wlroots@754f40f9

include/layers.h

@@ -17,6 +17,7 @@ struct lab_layer_surface {
 	struct wl_listener output_destroy;
 
 	struct wlr_box geo;
+	bool mapped;
 };
 
 void layers_init(struct server *server);