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.
diff --git a/src/xwayland/window-manager.c b/src/xwayland/window-manager.c
index 378daee..a2a12bd 100644
--- a/src/xwayland/window-manager.c
+++ b/src/xwayland/window-manager.c
@@ -460,6 +460,9 @@
 
 	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);