Does not obey QT_SCALE_FACTOR, hence breaks HiDPI

We are exporting `QT_SCALE_FACTOR=2` to switch into HiDPI mode.

However, KWin does not obey this.
In fact, looking at the source code, it seems to me it intentionally unsets this variable?

https://github.com/KDE/kwin/blob/9f41691684ffa2f42f09cfb8a24ec23fc146fe27/src/main_x11.cpp#L416

As a result, title bars and other window decorations do not scale; unlike the rest of Qt.

https://phabricator.kde.org/D20234 confirms the suspicion that it _intentionally_ does not follow `QT_SCALE_FACTOR` since

> Qt scaling and kwin on X11 is never going to work so it is disabled.

Why is it working for all of Qt but not for KWin?