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