High DPI icons are not scaled using smooth filtering

1. Create a QPixmap from image
2. Use setDevicePixelRatio(2.0) to signify this is a 2x (retina) pixmap
3. Make an icon from pixmap and draw on a non-retina (pixel ratio 1.0) screen. Note that the pixmap has artifacts

These artifacts appear to occur due to the use of the Qt::FastTransform when making the scaled pixmap. Possible solution: use Qt::SmoothTransform instead