Only turn off font hinting when really scaling

We don't want to turn off font hinting based on whether auto-scaling is on,
otherwise Qt Creator will look bad on a low-DPI screen. Instead, we turn it
off only if we have at least one screen that is scaled.

QHighDpiScaling::isActive() now means that there is actual scaling going on.
There is currently no need for the old meaning, so this change does not include
QHighDpiScaling::isEnabled().

Note that nothing can save us from the case where there is one high-DPI and
one low-DPI screen.  In that case we choose looking bad on the low-DPI screen
instead of looking like crap on the high-DPI screen. Also note that our font
system we doesn't allow us to change our minds on hinting later when screens
are plugged in or removed.

Change-Id: I14a4ec7a49f4ba74a4c74684c7b951d0a71b951d
Reviewed-by: Morten Johan Sørvig <morten.sorvig@theqtcompany.com>