Backport fix (2nd try): Fix highdpi conversion of QTabletEvent coordinates on xcb

David Koňaříkrequested to merge
dvdkon/qtbase:kde/5.15 into kde/5.15

Backports fix for QTBUG-77826, which is a pretty nasty bug breaking stylus pointing devices on Linux.

This is a second try after !60 (merged), another commit was backported to provide the necessary coordinate mapping functions.

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