bug#33729: 27.0.50; Partial glyphs not rendered for Gujarati with Harfbuzz enabled (renders fine using m17n)

[Khaled Hosny <dr.khaled.hosny@gmail.com>]

On Sat, Dec 22, 2018 at 05:42:43PM +0200, Eli Zaretskii wrote:
> > Date: Sat, 22 Dec 2018 17:15:09 +0200
> > From: Khaled Hosny <dr.khaled.hosny@gmail.com>
> > Cc: rgm@gnu.org, far.nasiri.m@gmail.com, behdad@behdad.org,
> > 	33729@debbugs.gnu.org, kaushal.modi@gmail.com
> > 
> > > There's a FIXME in the change you pushed (which I believe just
> > > repeats what was already in the previous version).  Ca you tell more
> > > about the problem we need to fix there?
> > 
> > We need a way to get Unicode composition and decomposition for the
> > a given character (implementing the uni_compose and uni_decompose
> > functions I deleted).
> 
> Yes, but what does that entail?  Are these compositions and
> decompositions defined by the Unicode UCD?  And how does Harfbuzz use
> the results for a given character?

Yes, the standard Unicode composition and decomposition. HarfBuzz uses
these during shaping (it prefers composed form for a given sequence if
supported by the font, and falls back to decomposed form otherwise).

> > I recall you suggested something earlier that I tried but couldn’t
> > get to work, the exact detail escapes me.
> 
> I probably suggested using the 'decomposition' property of a
> character, and perhaps als the facilities in ucs-normalize.el.

How can this be done from C.