[HarfBuzz] Issue with cursive attachment

[Khaled Hosny]

I’ve a problem with mixing cursive attachment lookups with RTL flag set
on some of them and not the others.

I’m attaching two minimal fonts showing this issue, the only different
between the good and bad fonts is that the bad font has a cursive
attachment lookup with RTL flag not set while all the others have it
set, and HarfBuzz seems to not to apply that lookup on the same word
where the other lookups are applied.

Testing with the word كمثل, the anchor between the 2nd and 3rd glyphs
(from right) is not applied in the bad font but applied in the good one.
Both DirectWrite/Uniscribe and Core Text apply it fine in both fonts
(though Core Text has other serious problems).

If I unset the RTL bit on all the cursive attachment lookups, it renders
correctly in HarfBuzz.

(If you are wondering why I’m doing this, it is because for some of my
cursive attachments I want rightmost glyph to stay in its natural
position and the other glyphs attached to it instead of having the
leftmost glyph stay in its natural position.)

Regards,
Khaled
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