Qt computes the scale factor relative to the primary screen. This means
we can support per-screen DPI on X11 by setting QT_SCREEN_SCALE_FACTORS
appropriately, while keeping Xft.dpi synced to the primary screen's DPI.
This means applications which don't read QT_SCREEN_SCALE_FACTORS use the
scale of the primary screen. This is a good compromise, especially for
those who either need fractional scaling or can't use wayland for various
reasons.
WIP because this doesn't track changes of the primary screen and I'm not
sure how to implement this the best way. The issue is that the
QT_SCREEN_SCALE_FACTORS management is not done by the backend, but only by
the KCM. Ideas welcome.