Treat math style a bit more like other font features

Treat math style a bit more like other font features

While math style (scriptstyle,...) is not really something that can be
set, it is useful for text subscripts and superscripts and therefore
it makes sense to handle it in some places.

With this change, style is still not a first class feature, but good
enough for now. In particular, it is taken into account in update().