[Gtk] Fix FastPangoAttrList.AddFontAttribute
[Gtk] Fix FastPangoAttrList.AddFontAttribute

There was an exception due to trying to pass the C# wrapper class to the
P/Invoke instead of the native handle.

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