[HarfBuzz] MarkAttachmentType lookup flag has no effect

[Khaled Hosny]

It seems that the (older) MarkAttachmentType lookup flag is not applied
by HarfBuzz, while the (newer) UseMarkFilteringSet flag works fine.

Here is the same font font once using MarkAttachmentType and once using
UseMarkFilteringSet with the same mark glyph class in both cases (and it
is the only one used), and if applied the final dots in the words تختة,
تخنة, تخئة, تخثة and تخٹة should be raised up whether vowel marks are
used or not.

http://khaledhosny.org/files/tmp/hussaini-nastaleeq_MarkAttachmentType.ttf
http://khaledhosny.org/files/tmp/hussaini-nastaleeq_UseMarkFilteringSet.ttf

Regards,
 Khaled