bug#33729: 27.0.50; Partial glyphs not rendered for Gujarati with Harfbuzz enabled (renders fine using m17n)

[Benjamin Riefenstahl]

Khaled Hosny writes:
> /* FIXME: from_string wants an ISO 15924 script tag here. */
>
> As we discussed earlier, hb_script_from_string() expects ISO 15924
> script tags, but char_script_table does not provide such tags (I don’t
> recall what it does provide exactly). We need a way to get ISO 15924
> script tags from Emacs.

The same mismatch also prevents Syriac text from actually shaping.
Syriac shaping works in m17n with the required setup in
composition-function-table and using the Meltho fonts.  With Harfbuzz it
doesn't work, unless I change "syriac" to "syrc" in charscript.el, just
for testing of course.

As a success story OTOH, Mandaic, using the Noto font, works OOTB ;-)

benny