Destroy render loop when QCoreApplication is cleaned up.

Doing it on 'aboutToQuit' meant we would clean it up whenever an
application ran QCoreApp::exec() which could be multiple times
per application run.

Change-Id: I59e06398a551ae7979e3832dff811937037fa106
Reviewed-by: Robin Burchell <robin+qt@viroteck.net>