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.

BUG: 432766