During construction of a QStyle, QApplication might not yet be
initialized, e.g. when calling QApplication::setStyle("macOS") before
constructing QApplication. In that case, we cannot access the platform
theme.
We don't just want to skip initializing the small font either though.
Store the smallSystemFont as a std::optional so that we can initialize
it once, when the first widget gets polished.
As a drive-by, remove the unused miniSystemFont variable.
Fixes: QTBUG-108047
Pick-to: 6.4 6.2
Change-Id: Id750770a563611fdbc6c7031fe102a99ea692be7
Reviewed-by: Samuel Gaist <samuel.gaist@idiap.ch>
2bdc027f5c