message: adhere to sgOnly settings for sg import/export
diff: @@ -55,6 +55,9 @@ inline void to_json(nlohmann::json &j, const Node &n)
  return;
  }
  + if (n.type() == NodeType::PARAMETER)
  + j["sgOnly"] = n.sgOnly();
  +
  if (n.value().valid() && (n.type() == NodeType::PARAMETER
  || n.type() == NodeType::TRANSFORM))
  j["value"] = n.value();
  @@ -82,6 +85,8 @@ inline OSPSG_INTERFACE NodePtr createNode(const nlohmann::json &j) {
  if (j.contains("children")) {
  for (auto &jChild : j["children"]) {
  auto child = createNode(jChild);
  + if (jChild.contains("sgOnly") && jChild["sgOnly"].get<bool>())
  + child->setSGOnly();
  if (j["type"] == NodeType::LIGHTS)
  n->nodeAs<Lights>()->addLight(child);
  else
  