Determine if we should use Qt scaling based on env var
KWindowSystem::platform() cannot be used before QApplication is
QCoreApplication::setAttribute needs to be called before QApplication is
We need a different method. I've used an explicit new env variable so
that some users can also opt in to use this when on X.
Plasma panels all work. Look to be native resolution.
Reviewers: Plasma, mart
Reviewed By: Plasma, mart
Subscribers: mart, plasma-devel
Differential Revision: https://phabricator.kde.org/D7200