more coherent render threading for now

Once again, conceptually this system should work, however it does not seem to
be the most efficient and it wouldn't surprise me if the multithreading was
eventually upgraded to an even more coherent system one day. However for
"core principles" this should be fairly decent.

app/render/backend/videorenderbackend.cpp

@@ -187,6 +187,7 @@ void VideoRenderBackend::Close()
 
   foreach (QThread* thread, threads_) {
     thread->quit();
+    thread->wait(); // FIXME: Maximum time in case a thread is stuck?
   }
   threads_.clear();