Use correct flag when checking whether a scene window is opaque

Summary:
Given that allclips contains opaque regions of higher windows, we should
check PAINT_WINDOW_TRANSLUCENT flag instead of PAINT_WINDOW_TRANSFORMED.

ba39d1f9 introduced that check. Neither
the commit message nor the associated review have any comments regarding
checking PAINT_WINDOW_TRANSFORMED, so given the following comment

    // Here we rely on WindowPrePaintData::setTranslucent() to remove
    // the clip if needed.

PAINT_WINDOW_TRANSFORMED is most likely a typo.

Reviewers: #kwin, graesslin

Reviewed By: #kwin, graesslin

Subscribers: graesslin, kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D18236