[REG 5.13.2->5.15.6] QT anti-aliasing mode does not respect operating system's settings

This bug is a regression. It was functioning correctly where it respected the operating system's anti-aliasing preference in QT 5.13.2 and earlier. Versions of QT after that in Windows and Linux began enforcing anti-aliasing. The issue still exists in Linux as well as of version QT 5.15.6 where the operating system's preference is ignored so it is a multi-platform regression not solely a Windows one.

It's an issue because some fonts are selected because they are hinted to not need anti-aliasing or are bitmapped, etc. Also the higher resolution a display is, the less and less anti-aliasing is desired. And some users get eyestrain from the blur effect of AA.

The attached PNG (QT 5.13.2 on the right) demonstrates the problem. In a browser the image itself will look a bit blurry but if you save the image to your desktop and view it at 100% scale in an image viewer, you will see the difference between the two halves clearly.

It is also causing an ugly UI issue in Linux where some areas of a program window will respect the operating system's setting (e.g. window titlebar) but not other areas (e.g. text inside the window itself).

Possibly related to https://bugreports.qt.io/browse/QTBUG-112503