[HarfBuzz] Interaction between default and non default features

[Khaled Hosny]

While trying to added a stylistic set to an Arabic font, I found that
they are applied after all default features no matter how the lookups
are ordered in the font.

I sort of understand why it is done this way, but it strikes me as it
renders stylistic sets near useless when any complex substitution is
needed. In my case I want to convert all regular yeh letters to yeh
barree form, but it have to be done before any other substitution to
allow for contextual substitution to take place, doing it after
contextual substitution will require much more complex rules.

Here is the font for testing:
http://khaledhosny.org/files/tmp/hussaini-nastaleeq_ss01.ttf

(with something like “ى ىى لى فى”)

So I’m asking if the current behaviour is “final” and no intentions to
change it, and if anyone knows how other implementations deal with this
(the Uniscribe backed seems to ignore the stylistic set completely, so I
don’t know how to test this).

Regards,
 Khaled