Follow scale of drag surfaces
Summary:
Similar to regular cursors a drag surface can have a scale.
The buffer "rect" should be in logical pixels at which point QPainter
will automatically handle everything.
CCBUG: 421395
Reviewers: KWin, zzag, apol
Reviewed By: KWin, zzag, apol
Subscribers: kwin
Tags: KWin
Differential Revision: https://phabricator.kde.org/D29672