[HarfBuzz] Order of combining diacriticals

[Eli Zaretskii]

> From: Behdad Esfahbod <behdad at behdad.org>
> Date: Fri, 14 Jun 2019 11:34:17 -0700
> Cc: Khaled Hosny <dr.khaled.hosny at gmail.com>, 
> 	"harfbuzz at lists.freedesktop.org" <harfbuzz at lists.freedesktop.org>
> 
> On Thu, Jun 13, 2019 at 2:18 AM Eli Zaretskii <eliz at gnu.org> wrote:
> 
>    . For fonts that have no 'hebr' features, Emacs performs
>      substitution of known precomposed characters before it invokes the
>      shaping engine.  In this case, it substituted U+FB31 for the
>      sequence U+05D1,U+05BC, and passed the sequence U+FB31,U+05B0 to
>      HarfBuzz.
> 
> You should remove all such hacks.

I understand that for HarfBuzz they are probably not needed, if the
necessary functions for accessing the glyphs are provided (something
that might not be true on Windows, where we don't use Freetype
directly).  But Emacs also has other font backends, which are not as
capable.

In any case, this particular situation uncovered a subtle bug in how
Emacs uses the information provided by HarfBuzz, so it was a Good
Thing we did have this particular hack.

Thanks.