message: Fix Node::add
  if replace child, update its parents
  only markAsModified if actually different than before
diff: @@ -149,6 +149,11 @@ namespace ospray {
  void Node::add(NodePtr node, const std::string &name)
  {
  + if (hasChild(name)) {
  + if (properties.children[name] == node)
  + return;
  + properties.children[name]->removeFromParentList(*this);
  + }
  properties.children[name] = node;
  node->properties.parents.push_back(this);
  markAsModified();
  