bug#57072: [BUG] update-glyphless-char-display and variation selectors

[Eli Zaretskii]

> From: Axel Svensson <svenssonaxel@gmail.com>
> Date: Tue, 9 Aug 2022 22:33:40 +0200
> Cc: 57072@debbugs.gnu.org
> 
> > Now, VS16 is almost always composed with preceding characters, so I
> > think you can only see it as acronym if you deliberately force Emacs
> > not to compose it, e.g. by preceding it with U+20DD COMBINING
> > ENCLOSING CIRCLE, or precede it and follow it by U+200C ZERO WIDTH
> > NON-JOINER, or disable auto-composition-mode.
> 
> - Preceding it with U+20DD still produces the empty box
> - Preceding it and following it by U+200C still produces the empty box
> - Disabling auto-composition-mode produces the "VS16" acronym.

Yes, I think this is because of the special composition rules we have
for VS16 (which are required to display Emoji sequences correctly).