When font antialising is disabled, the created alpha maps will be Mono
format. Make sure that the created QImage is valid by ensuring that
it has a color table of the right size.
Issue was earlier handled by a4e2f2e687,
but that will be partially reverted because of compatibility break.
Task-number: QTBUG-50745
Change-Id: I7c521288469ae94805a3326644771270d97ab979
Reviewed-by: Konstantin Ritt <ritt.ks@gmail.com>
Reviewed-by: Eskil Abrahamsen Blomfeldt <eskil.abrahamsen-blomfeldt@qt.io>