bug#39799: 28.0.50; Most emoji sequences don’t render correctly

[Eli Zaretskii]

> Date: Fri, 28 Feb 2020 22:25:44 +0200
> From: Eli Zaretskii <eliz@gnu.org>
> Cc: 39799@debbugs.gnu.org
> 
> Hmm... it's possible I was confused, and the functions I mentioned are
> unrelated to variation selectors.  To see if that's so, try to
> configure composition-function-table to display such sequences as
> composed characters, and see what happens when you use a proper font
> (e.g., the one with which Gedit displays the variations).

Looking into this some more reveals that we already have
composition-function-table set up for variation selectors, see the end
of lisp/language/japanese.el.  Not sure why it's in japanese.el, but
the code doesn't seem to be specific to Japanese characters, unless
I'm missing something.  So some debugging is required to understand
why we don't display sequences with variation selectors as intended.
Maybe DejaVu Sans doesn't support that?  What if you try

  emacs -Q -fn Noto Color Emoji"

does Emacs built with HarfBuzz then display the variation sequences as
expected?