Fix not having a context when cleaning up on Windows

The same old issue surfaces in the windows render loop too.
The basic render loop is already fixed, apply a similar patch
to the windows one too.

Task-number: QTBUG-42213
Change-Id: I07068315f5164014e329b8ce061947c97ae9da61
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@theqtcompany.com>
Reviewed-by: Sean Harmer <sean.harmer@kdab.com>