Enabling "curs" font feature doesn't seem to enable it, in the attached
example I enabled "curs" for the first line and disabled it for the
second one, but it doesn't seem to get applied (the same output in the 2
lines). 
The font is here
http://www.khaledhosny.org/files/pub/SimplifiedNaskh/SimplifiedNaskh.ttf

-- 
 Khaled Hosny
 Arabic localizer and member of Arabeyes.org team