disable client's scene node after create it

will be enabled when setting up its monitor and arrange it

Bug: #306

dwl.c

@@ -1462,6 +1462,7 @@ mapnotify(struct wl_listener *listener, void *data)
 
 	/* Create scene tree for this client and its border */
 	c->scene = wlr_scene_tree_create(layers[LyrTile]);
+	wlr_scene_node_set_enabled(&c->scene->node, 0);
 	c->scene_surface = c->type == XDGShell
 			? wlr_scene_xdg_surface_create(c->scene, c->surface.xdg)
 			: wlr_scene_subsurface_tree_create(c->scene, client_surface(c));