Determine if we should use Qt scaling based on env var
Summary:
KWindowSystem::platform() cannot be used before QApplication is
constructed.
QCoreApplication::setAttribute needs to be called before QApplication is
constructed.
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.
Test Plan:
Logged in.
Plasma panels all work. Look to be native resolution.
Reviewers: Plasma, mart
Reviewed By: Plasma, mart
Subscribers: mart, plasma-devel
Tags: Plasma
Differential Revision: https://phabricator.kde.org/D7200