bug#31315: wrong font encoding for fallback font

[Eli Zaretskii <eliz@gnu.org>]

> Date: Thu, 03 May 2018 21:05:28 +0200 (CEST)
> Cc: handa@gnu.org, 31315@debbugs.gnu.org
> From: Werner LEMBERG <wl@gnu.org>
> 
> > May I invite you to work on revisiting the design and implementation
> > of the Emacs font-look up facilities, and on modernizing them?  I'm
> > afraid we didn't have an active developer in this area for several
> > years, and I fear that we will stagnate (or already are stagnating).
> 
> Alas, my Elisp knowledge is ... well ... not impressive.  Basically,
> I'm just an Emacs user, not an Emacs developer.

Almost all of the relevant code is in C, not in Lisp.

> With Xft, there is no possibility for improvement IMHO.  The probably
> best choice is to switch to Pango for font access (in case you don't
> do that already).

There are several back-ends besides Xft, the most advanced being
xftfont.c.  They all didn't see any serious development for the past
several years.  And yes, acquiring new back-ends is also a worthy
goal.

All of that requires a level of expertise that IMO we currently don't
have.

> An Emacs charset essentially triggers a language and script setting,
> and those two parameters can be passed to Pango, AFAIK.

Emacs handles charsets and scripts separately.  A script is matched
against OTF/TTF features of fonts to make sure the necessary shaping
features required by a script are supported.