[HarfBuzz] Arabic Presentation Forms-A in fallback shaping

[Khaled Hosny]

Hi Behdad,

While comparing the fallback shaping ICU-based XeTeX with the
HarfBuzz-based one, I noticed that ICU uses the Arabic Presentation
Forms-A ligatures if the font have them unlike HarfBuzz. It is not a big
issue (I don’t even have such font, I striped OT tables from fonts to
test), but it would be nice to have.

Here are the two fonts I tested:
http://khaledhosny.org/files/tmp/kacstone-noot.ttf (no forms-A)
http://khaledhosny.org/files/tmp/amiri-regular-noot.ttf (has forms-A)

Regards,
Khaled