[HarfBuzz] Arabic vowels and shaping - incomplete behavior

[Tom Hacohen]

Hey,

Is it just me, or do vowels break shaping in Arabic? If so, what should
be done to fix it? I can't think of a way of solving it, maybe only
removing the vowels, shaping without them and then adding previously
shaped glyphs, and then vowels and then the glyphs again and reshaping,
but that's annoying....

Any ideas? Example text that works in gtk but doesn't with my
implementation:

ضَلُّو

Thanks,
Tom.