Make sure texture factories are delete before libraries are unloaded.

Because scene graph adaptations and custom implementations can
instantiate QQuickTextureFactories, we need to make sure that
their destructors are called before their libraries are unloaded
to avoid crashes on shutdown.

Change-Id: I17d38e2909aabcb4ea7bf4eecc29df86c8479fdb
Reviewed-by: Robin Burchell <robin+qt@viroteck.net>