Update root order on layer change

When the layer of a CanvasLayer changes, the order of roots needs to be recalculated.

(cherry picked from commit 8d60e8b5d4)

scene/main/canvas_layer.cpp

@@ -36,6 +36,7 @@ void CanvasLayer::set_layer(int p_xform) {
 	layer = p_xform;
 	if (viewport.is_valid()) {
 		VisualServer::get_singleton()->viewport_set_canvas_stacking(viewport, canvas, layer, get_position_in_parent());
+		vp->_gui_set_root_order_dirty();
 	}
 }