Stop osg viewer threading before destructing engine

To avoid rendering while engine parts are destructing.

apps/openmw/engine.cpp

@@ -970,6 +970,8 @@ void OMW::Engine::go()
     // Save user settings
     settings.saveUser(settingspath);
 
+    mViewer->stopThreading();
+
     Log(Debug::Info) << "Quitting peacefully.";
 }