Scale offset as well on QBackingStore flush

Also use the target window ("win") as context instead
of d->window.

Change-Id: I5b0fa3bb857526e4a4bc9c5e44670593da1dfe7c
Task-number: QTBUG-50487
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@theqtcompany.com>
Reviewed-by: Morten Johan Sørvig <morten.sorvig@theqtcompany.com>
Reviewed-by: Paul Olav Tvete <paul.tvete@theqtcompany.com>