Make use of the device pixel ratio in the QIcon paint method so the @nx
hi-dpi pixmaps are selected when appropriate when painting to a
QPainter.
Pick-to: 6.0
Fixes: QTBUG-90042
Change-Id: I53995a2285ef879e3c4fddb9f8da702e256a260f
Reviewed-by: Eirik Aavitsland <eirik.aavitsland@qt.io>