osd: prevent preview outlines from overlapping OSD in first output

Following code didn't consider multi-monitor setup:

  wlr_scene_node_raise_to_top(&server->menu_tree->node);
  wlr_scene_node_raise_to_top(&output->osd_tree->node);

...because when executed twice, server->menu_tree is raised above
output->osd_tree for the first output. This resulted in a scene tree like
(higher to lower):

- output->osd_tree (for 2nd output)
- output->server->osd_state.preview_outline (placed just above menu_tree)
- sever->menu_tree
- output->osd_tree (for 1st output)

So the previous outline for window switcher is placed above the OSD in the
first output.

src/server.c

@@ -580,11 +580,6 @@ server_init(struct server *server)
 #if HAVE_XWAYLAND
 	server->unmanaged_tree = wlr_scene_tree_create(&server->scene->tree);
 #endif
-
-	/*
-	 * menu_tree is moved to top in new_output_notify() when layer-shell
-	 * layers are positioned
-	 */
 	server->menu_tree = wlr_scene_tree_create(&server->scene->tree);
 
 	workspaces_init(server);