Scene clear fix

src/core/scene/glscene.cpp

@@ -108,6 +108,9 @@ void Scene::clear() {
 	emit __destroyed();
 	if (destroying) return;
 	emit treeChanged();
+	mat_changed = lights_changed = true;
+	need_reload_materials        = true;
+	tree_struct_changed          = true;
 }