Perform a @Nx image file lookup when loading pixmaps.
Make drawBackgroundImage() handle high-dpi pixmaps,
here the layout calculations needs to be in device-
independent pixels
Fixes: QTBUG-36825
Change-Id: I61e6f53c59f61f3bd88c34a036349e51e8c8ad92
Reviewed-by: Ulf Hermann <ulf.hermann@qt.io>
Reviewed-by: Alex Blasche <alexander.blasche@qt.io>