message: fix bug with log/error output callback being installed too often
diff: @@ -101,7 +101,7 @@ namespace ospray {
  utility::getEnvVar<std::string>("OSPRAY_LOG_OUTPUT");
  auto dst = OSPRAY_LOG_OUTPUT.value_or(
  - getParam<std::string>("logOutput", "none")
  + getParam<std::string>("logOutput", "")
  );
  if (dst == "cout")
  @@ -115,7 +115,7 @@ namespace ospray {
  utility::getEnvVar<std::string>("OSPRAY_ERROR_OUTPUT");
  dst = OSPRAY_ERROR_OUTPUT.value_or(
  - getParam<std::string>("errorOutput", "none")
  + getParam<std::string>("errorOutput", "")
  );
  if (dst == "cout")
  