When resizing, the device pixel ratio needs to be taken into account.
Otherwise the area for Xaml viewer will be misplaced and/or wrongly
scaled.

Change-Id: Id2dd34c2dd29f5582a9f3536e6394c7e49eddf94
Task-number: QTBUG-53405
Reviewed-by: Oliver Wolff <oliver.wolff@qt.io>