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

[Axel Svensson]

> 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.