layers: don't send configure events in unmap handler

Alternative to 7bf08af which was reverted in the previous commit.

7bf08af fixed the problem that layer-shell clients are terminated when
it's unmapped, by sending configure events in node-destroy handler
rather than in unmap handler. But it caused a UAF bug when an output
with layer-shell clients is destroyed.

So this patch fixes the original issue by simply skipping the surface in
arrange_one_layer() if it's being unmapped.

include/layers.h

@@ -14,6 +14,8 @@ struct lab_layer_surface {
 	struct server *server;
 
 	bool mapped;
+	/* true only inside handle_unmap() */
+	bool being_unmapped;
 
 	struct wl_listener map;
 	struct wl_listener unmap;