message: remove old instances from world in RenderScene
  we were only called setParam on the world if we had instances to
  provide, which normally has the behavior of replacing whatever was
  previously in the world. However if we had no instances, nothing would
  happen, causing old objects to still be rendered
diff: @@ -239,6 +239,8 @@ namespace ospray::sg {
  {
  if (!instances.empty())
  world.setParam("instance", cpp::CopiedData(instances));
  + else
  + world.removeParam("instance");
  }
  } // namespace ospray::sg
  