Add public API for enabling QT_WIDGETS_HIGHDPI_DOWNSCALE

a.k.a. stabilize QT_WIDGETS_HIGHDPI_DOWNSCALE

The downscale feature improves support for fractional scale factors by rendering the UI at the next higher integer scale factor, and then downscaling the backing store frame buffer to the target scale factor.

For instance, a Qt application running on a Windows 175% scale display will render at DPR 2 instead of 1.75 when QT_WIDGETS_HIGHDPI_DOWNSCALE is enabled.

Tasks:

• Deduplicate "enable" code (https://codereview.qt-project.org/c/qt/qtbase/+/581269/)
• Determine type of API
  • Qt::AA_ application attribute
  • Qt::WA_ widget attribute
  • both/other?
• Determine how the QT_WIDGETS_RHI dependency should be handled (auto enable, or add API for that as well, or ...)
• Add auto-test to prevent regressions
• Write Documentation
• Write Blog