Comment 0 for bug 1610208

Qt 5.6 supersedes QT_DEVICE_PIXEL_RATIO with three variables:
- QT_SCALE_FACTOR, which works the same way.
- QTSCREEN_SCALE_FACTORS, which lists values per monitor.
- QT_AUTO_SCREEN_SCALE_FACTOR, which automatically picks a value for each monitor.

Documentation needs to take these new variables into account and we need to start using the new variables where applicable (the old one still works, but triggers warnings with Qt 5.6).