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