Without an override for logicalDpi() the base class implementation
would use the geometry of the screen to figure out the DPI, and end
up with ~100, which combined with a 96DPI base logical DPI would
give a wrong scale factor.
Change-Id: I68aecce44d2ee672c7b707dfe5444af8f551e961
Reviewed-by: Morten Johan Sørvig <morten.sorvig@qt.io>