Correctly check forcebackgroundcontrastrule
Summary:
WindowBackgroundContrastRole property is a bool or undefined.
QVariant::isValid() returns true for QVariant(false) which is not what
we want for this code.
toBool on an invalid QVariant will return false.
Test Plan:
NA. Was based on code review.
To get a bug you'd have to PresentWindows/Slide effect manipulate a sliding popup, which
AFAIK doesn't happen.
Reviewers: KWin
Subscribers: kwin
Tags: KWin
Differential Revision: https://phabricator.kde.org/D13478