[HarfBuzz] Placement of arabic diacritics over 3 component ligature is incorrent

[Khaled Hosny]

Going through old Pango bugs to see if there is anything HarfBuzz
related, I found this one which is still reproducible with HarfBuzz
(marks over 2nd component of لله are placed over the 3rd one, marks over
the other two components are placed correctly):

https://bugzilla.gnome.org/show_bug.cgi?id=437633

Regards,
 Khaled