Investigate UI rendering at fractional scale factors

We're looking to make Qt 6 use the native UI scale directly (without rounding), which means that Qt GUIs may be rendered at e.g. 1.25x and 1.75x.

This change was implemented in https://codereview.qt-project.org/c/qt/qtbase/+/273956 .

This requires that Qt applications are actually usable at fractional scale factors, and that any visual artifacts are not too jarring. Running at an integer scale factor will give the best visual results, which is something we should consider pointing out in the documentation.

App developers can enable rounding using  the QGuiApplication::setHighDpiScaleFactorRoundingPolicy() API.

Test cases:

• Qt Quick Controls 2 (should be OK)
• Qt Wigets styles: (examples/widgets/gallery)
  • Fusion style at 1.25x:

• Some missing frame lines, otherwise OK.
  
  • Vista style