bug#41645: 27.0.91; Combining Grapheme Joiner (#x34f) gui artifacts

[Eli Zaretskii <eliz@gnu.org>]

> From: David Fussner <dfussner@googlemail.com>
> Date: Tue, 2 Jun 2020 14:45:50 +0100
> Cc: Eli Zaretskii <eliz@gnu.org>, 41645@debbugs.gnu.org
> 
> A couple of data points, in case they're helpful:
> 
> On 27.0.91 _unpatched_, I see the artifact whenever the font of the
> CGJ is different from that of the glyph before it, no matter which
> script I'm using. When the font of the CGJ and the previous glyph are
> the same, I don't see the artifact, except in Hebrew, where it's still
> present. C-u C-x = displays the CGJ on its own, as a separate glyph,
> whenever it's used in Hebrew and also whenever its font doesn't match
> that of the glyph before it. When the font does match, in Latin or
> Greek script, the cursor doesn't stop on the CGJ, and C-u C-x = shows
> it as composed with the previous character.

The visual artifact is due to the font you have that is used to
display CGJ, a font that is different from the one used for displaying
the surrounding text.  More about this in a separate message.

Regardless, the effect of CGJ on Hebrew points that you wanted to
have---the details can be seen in this URL:

  https://en.wikipedia.org/wiki/Combining_Grapheme_Joiner

did not work in Emacs, because Hebrew has its own composition rules,
and those rules didn't allow for the CGJ among the points.  I've now
added that, so if you rebuild the master branch, and use a font that
can support both Hebrew characters and CGJ, you should see the effect
of that on reordering the Hebrew points.