Only try to cleanup the resources if we know we are called from the right
thread.

releaseGraphicsResources can be called from 2 places:
- Scene3DCleaner when using Scene3D (when closing the window)
- AspectThread when the RenderAspect gets unregistered

In configurations where the context lives in the main thread (Scene3D +
single threaded render loop / ANGLE), the RenderAspect gets unregistered
before the Scene3DCleaner has had time to call releaseGraphicsResources.

This means that we won't release the resources ourselved and that the
driver will have to do that for us.

This has been fixed properly in 5.14 with the AspectThread removal.

Task-number: QTBUG-60971
Change-Id: I49c1c2f74ad09e7162b988f87bac65863f8490d8
Reviewed-by: Sean Harmer <sean.harmer@kdab.com>