Rename internal root canvas group to start with underscore

scene/main/canvas_item.cpp

@@ -222,7 +222,7 @@ void CanvasItem::_enter_canvas() {
 		RenderingServer::get_singleton()->canvas_item_set_parent(canvas_item, canvas);
 		RenderingServer::get_singleton()->canvas_item_set_visibility_layer(canvas_item, visibility_layer);
 
-		canvas_group = "root_canvas" + itos(canvas.get_id());
+		canvas_group = "_root_canvas" + itos(canvas.get_id());
 
 		add_to_group(canvas_group);
 		if (canvas_layer) {