[HarfBuzz] Support for Stylistic Sets

[Eli Zaretskii]

> Date: Sat, 14 Sep 2019 18:13:25 +0100
> From: Richard Wordingham <richard.wordingham at ntlworld.com>
> 
> I think it's safe to specify the use of unsupported features, in which
> case this is a luxury feature.

you mean, specifying an unsupported feature will not cause hb_shape to
fail, but instead just use the nominal glyphs?

> One complication is that features are provided by a font on a (per
> script) per language basis.

Why is that a complication?  The user who requests the feature should
do so only for text of a suitable script, no?

> For example, my Da Lekh font provides feature ss19 for the default
> language, but not for Lao, Tai Lü or 'Shan'. In this font, Feature
> ss19 means apply Lao style, and that is applied automatically if the
> font is told it is being used for Lao. It would be a bit off to tag
> aerated Pali text as Lao just to get a Lao style. Aerated Pali has
> different line-breaking rules to Lao, which is written without visible
> word separation.

Emacs currently leaves it to HarfBuzz to guess the language, so I
don't think this is an issue.