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>

src/gui/painting/qbackingstore.cpp

@@ -106,7 +106,8 @@ void QBackingStore::flush(const QRegion &region, QWindow *win, const QPoint &off
     }
 #endif
 
-    d_ptr->platformBackingStore->flush(win, QHighDpi::toNativeLocalRegion(region, d_ptr->window), offset);
+    d_ptr->platformBackingStore->flush(win, QHighDpi::toNativeLocalRegion(region, win),
+                                            QHighDpi::toNativeLocalPosition(offset, win));
 }
 
 /*!