xwm: Ignore map request for already mapped window

If a client sends another map request before the server has seen our
reply to the first map request event, we might get a map request for an
already mapped window.  Just ignore that.

src/xwayland/window-manager.c

@@ -460,6 +460,9 @@ weston_wm_handle_map_request(struct weston_wm *wm, xcb_generic_event_t *event)
 
 	window = hash_table_lookup(wm->window_hash, map_request->window);
 
+	if (window->frame_id)
+		return;
+
 	weston_wm_window_read_properties(window);
 
 	weston_wm_window_get_frame_size(window, &width, &height);