Fix current content in the destroy of the Scene

Fix the error:
> QOpenGLVertexArrayObject::destroy() failed to restore current context

Polyhedron/demo/Polyhedron/Scene.cpp

@@ -1928,6 +1928,7 @@ void Scene::removeViewer(Viewer_interface *viewer)
   if(viewer->property("is_destroyed").toBool())
     return;
 
+  viewer->makeCurrent();
   vaos[viewer]->destroy();
   vaos[viewer]->deleteLater();
   vaos.remove(viewer);