Qt APIs and features do not provide a way to get non-integer DPI scaling without artifacts

Qt APIs and features do not provide a way to get correct DPI scaling without artifacts.

Currently available APIs result in either incorrect scaling or painting artifacts as shown with the attached screenshots incorrect-scaling,png and artifact.png.

The painting artifacts occur when using non-integer scaling. The examples here are using 150% scaling.

The attached code can be built with defines showing the issue, and a FIX_SCALING define which proposes one solution based on changing the API for Qt 6 and is shown in correct-scaling.png. It is further described here:

 http://lists.qt-project.org/pipermail/development/2017-September/030935.html

Any complete/correct solution will do. The QCoordinate is just a proposal.