During the QPA refactoring, the custom font substitution list support
was lost by ignoring the font request's fallbackFamilies member
when the multi font engine gets created/initialized.
If fallbackFamilies is not empty, it should be prepended to the
font database default fallback families list. Also respect the
custom fallback families list in the cache key to avoid picking
a multi font engine with wrong fallbacks list.
Task-number: QTBUG-36628
Change-Id: Ie2b84b3a397bee4816f421cddf76a5375829f13a
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>
Reviewed-by: Lars Knoll <lars.knoll@digia.com>
800232e1d3