examples: Fix compositor-examples

Due to the way the wlr_output API was changed, these examples would
never get a frame event to start the rendering loop. We now commit the
outputs to start it.

(cherry picked from commit 2fea2fced8)

examples/multi-pointer.c

@@ -171,6 +171,8 @@ static void new_output_notify(struct wl_listener *listener, void *data) {
 			cursor->cursor->y);
 	}
 	wl_list_insert(&sample->outputs, &sample_output->link);
+
+	wlr_output_commit(output);
 }
 
 static void keyboard_key_notify(struct wl_listener *listener, void *data) {