QWidgetBackingStore::markDirty(): Clamp dirty region to window size.

Graphics effects may exceed the window size, causing platform
backing store operations to fail (see QWidgetPrivate::effectiveRectFor()).

Task-number: QTBUG-49785
Change-Id: Iff16da599397d19acb86010fe7023f3ce15b6d6f
Reviewed-by: Morten Johan Sørvig <morten.sorvig@theqtcompany.com>