D12405: [WIP] Per-screen scale factors on X11 using QT_SCREEN_SCALE_FACTORS

[Fabian Vogt]

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REVISION SUMMARY
  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.

TEST PLAN
  VM with multihead QXL. I can now assign a per-screen scale in the KCM
  and after a relogin applications scale correctly based on the screen they're on.

REPOSITORY
  R104 KScreen

BRANCH
  perscreenxcb

REVISION DETAIL
  https://phabricator.kde.org/D12405

AFFECTED FILES
  kcm/src/outputconfig.cpp
  kcm/src/scalingconfig.cpp
  kcm/src/scalingconfig.h
  kcm/src/widget.cpp
  kcm/src/widget.h

To: fvogt, #plasma
Cc: plasma-devel, ragreen, Pitel, ZrenBot, lesliezhai, ali-mohamed, jensreuterberg, abetts, sebas, apol, mart
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