Ending a QThread can cause deadlocks

In QEventLoop::exit(int returnCode) there is code with a comment related to QTBUG-70185 which calls:

emscripten_force_exit(returnCode);

It seems this is called wrongly when a QThread with QEventLoop terminates. If this happens this is delegated to the main thread and in our case causes a deadlock in cleanup code.

IMHO a background thread should never call emscripten_force_exit.