output: destroy output after ensuring it is disabled

This became necessary after the removal of the wlr_output_damage object.
Before, the wlr_output_damage destroy signal fired disabling the output,
then after the handle_destroy function would be called.

Fix this to stop on an assert.

sway/desktop/output.c

@@ -206,11 +206,12 @@ static void update_output_manager_config(struct sway_server *server) {
 static void handle_destroy(struct wl_listener *listener, void *data) {
 	struct sway_output *output = wl_container_of(listener, output, destroy);
 	struct sway_server *server = output->server;
-	output_begin_destroy(output);
 
 	if (output->enabled) {
 		output_disable(output);
 	}
+
+	output_begin_destroy(output);
 
 	wl_list_remove(&output->link);