bug#50177: Support U+20DD COMBINING ENCLOSING CIRCLE

[Eli Zaretskii]

> From: Lars Ingebrigtsen <larsi@gnus.org>
> Cc: 積丹尼 Dan Jacobson <jidanni@jidanni.org>,
>   50177@debbugs.gnu.org
> Date: Tue, 24 Aug 2021 17:30:13 +0200
> 
> We have a number of open bug reports in this area.  At least 44784,
> 23292, 17739, which may or may not be the same underlying problem.
> (Probably different issues, really.)

44784 and 23292 are due to a problematic font (DejaVu Sans Mono).
17739 -- not clear.

> I haven't looked at the machinery here at all -- is there a fundamental
> reason why Emacs can't combine glyphs from different fonts?

The basic reason is that glyphs from different fonts cannot combine
well because they were designed to look differently, and so offsets
don't match.  That is almost certainly the reason when we use our
fallback composition code in composite.el.  I'm less sure about modern
shaping engines like HarfBuzz -- we should ask their developers to be
sure; feel free to open an issue/question on their GitHub.

CC'ing Handa-san, in the hope that he could explain better why we
disallow character composition from different fonts.