[HarfBuzz] Exposing attachment tree / Arabic joining to shaping clients

[Khaled Hosny]

> Not sure how this can be explained, maybe someone has attempted to prevent some fonts to get Kashida justification and maybe the detection was font based, the reason it is still failing for IranNastaliq but not for Amiri, but in any case it is imperfect.

Amiri has a zero-width kashida glyph (that gets mapped to from camp, Then replace it by actual Kashida using GSUB), a hack I implemented to prevent Kashida justification in LibreOffice (why I also made LibreOffice check that font has a Kashida glyph with +ve width b before it tries to do kashida justification). I can imagine IE doing a similar check. You can try with Aref Ruqaa font which does a similar hack.