message: clean shutdown when choosing 'App'-->'Quit'
diff: @@ -559,10 +559,8 @@ namespace ospray {
  if (ImGui::MenuItem("Take Screenshot"))
  saveScreenshot("ospexampleviewer");
  - if (ImGui::MenuItem("Quit")) {
  - renderEngine.stop();
  - std::exit(0);
  - }
  + if (ImGui::MenuItem("Quit"))
  + exitRequestedByUser = true;
  ImGui::EndMenu();
  }
  