[HarfBuzz] Arabic GSUB changes

[Behdad Esfahbod]

Hi Khaled, others,

I just pushed out some code unifying the GSUB contextual input matching with
the ligature matching. I appreciate testing, since that part of the code is
very delicate.

Next I'm going switch Arabic fallback shaping to a synthesized table (like the
patch from Jonathan a while ago) so we can share logic with GSUB proper (and
even get per-ligature-component fallback mark positioning eventually!).

Cheers,
behdad