decoration: Fix rendering issues with scale factors < 1
QPainter::setWindow() doesn't work as we expect if the device pixel ratio of the paint device is less than 1, for example 0.5 or 0.75.
QPainter only allows the effective device pixel ratios that are greater than or equal to 1. This restriction probably has to be lifted.
For the time being, this change introduces a helper function that can be used to determine the scale factor by which QPainter::window() must be multiplied.
Edited by Vlad Zahorodnii