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

[Eli Zaretskii]

> Date: Fri, 14 Dec 2018 15:42:49 +0200
> From: Eli Zaretskii <eliz@gnu.org>
> Cc: behdad@behdad.org, kaushal.modi@gmail.com, 33729@debbugs.gnu.org,
>       far.nasiri.m@gmail.com
> 
> Mohammad, do you need help in comping up with a solution?  There's
> otf-script-alist (see fontest.el), but it goes in the opposite
> direction.  We could use rassq (Frassq in C) to find the OTF script
> tag by its Emacs symbol (which is returned by indexing into
> Vchar_script_table), by looking in otf-script-alist.
> 
> Or maybe you prefer a separate data structure, not limited to the OTF
> tags?

After some thinking, my conclusion is that we should import the
ISO 15924 database from https://unicode.org/iso15924/, use a script
similar to admin/unidata/blocks.awk to generate an alist from it that
maps Emacs script names to ISO 15924 tags, and then access that alist
from uni_script to get the correct script information to Harfbuzz.

Patches implementing that are welcome.