[HarfBuzz] Selecting fonts for HarfBuzz

[Eli Zaretskii]

> From: Behdad Esfahbod <behdad at behdad.org>
> Date: Tue, 4 Jun 2019 12:05:00 -0700
> Cc: "harfbuzz at lists.freedesktop.org" <harfbuzz at lists.freedesktop.org>
> 
> We assume fonts support shaping.  Ie. we don't have a way to check for font suitability for correct shaping.

I understand, thanks.  I wasn't asking how to do that with HarfBuzz, I
was asking what font features should my font matching function examine
to make sure the font will "support shaping" in the HarfBuzz sense.
Features that can be tested without actually shaping some text, of
course, i.e. without actually opening the font and using it.