[Mac] fix FontTextAttribute conversion
[Mac] fix FontTextAttribute conversion

The GetBackend(Font) returns FontData which has
a Font property and is is not a font itself.

antmicro/xwt,directhex/xwt,mminns/xwt,steffenWi/xwt,TheBrainTech/xwt,akrisiun/xwt,lytico/xwt,iainx/xwt,residuum/xwt,mono/xwt,cra0zy/xwt,hwthomas/xwt,hamekoz/xwt,mminns/xwt