[libkwineffects] Don't mark a painted window as transformed twice

Summary:
WindowPrePaintData::setTransformed sets PAINT_WINDOW_TRANSFORMED,
so `data.mask |= PAINT_WINDOW_TRANSFORMED;` is redundant.

Test Plan: Minimized a window, haven't noticed visual glitches.

Reviewers: #kwin, davidedmundson, graesslin

Reviewed By: #kwin, davidedmundson, graesslin

Subscribers: kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D17565