D29569: Fix computing display geometry on multi-monitor HiDPI setups on X11

[Victor Dodon]

printesoi added reviewers: davidedmundson, KWin.

REPOSITORY
  R278 KWindowSystem

REVISION DETAIL
  https://phabricator.kde.org/D29569

To: printesoi, davidedmundson, #kwin
Cc: kde-frameworks-devel, LeGast00n, cblack, michaelh, ngraham, bruns