QT_WIDGETS_HIGHDPI_DOWNSCALE is broken on Wayland

Looks like the fix for QTBUG-111102 has fixed the bug only on X11 while on Wayland I see that the picture is not scaled (i.e. painted like if the scaling was 1x while I have 1.25x) while input events are scaled right. I also see graphical artifacts when content updates. My guess is the fix does wrong calculations when the platform uses device-independent pixels.

Although maybe it's not related to QTBUG-111102 given that 6.5.0 is the first version with fractional scaling support on Wayland and it comes with broken QT_WIDGETS_HIGHDPI_DOWNSCALE, so it's unknown whether it worked at all in the past.

A compositor with fractional-scale-v1 support is required to see this bug (e.g. latest Plasma).