Turn off font hinting when active highdpi scaling

Font hinting is done in the Qt coordinate system, and ends up
looking very wrong if the painter is scaled. Now that high-DPI
scaling is cross platform, we can move the responsibility for
turning off hinting out of the platform plugin.

Note that we cannot change our minds later, since Qt does not have
a reference to all objects that perform text layout. Therefore, we
turn off hinting if and only if there is at least one screen with
scaling enabled at application startup. If this is not good enough
for you, you should disable scaling and support resolution independence
manually.

Task-number: QTBUG-46615
Change-Id: I15fa09aa1e952cf3abeb2d8c26f13ad1f1cbf9bf
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@theqtcompany.com>