[Fontconfig] No FC_FONT_FEATURES?

[Lawrence D'Oliveiro]

Seems like the FC_FONT_FEATURES pattern attribute is not implemented. I
was trying to implement a version of the collect_opentype_fonts script
from this repo <https://github.com/ldo/harfpy_examples>, only using
Fontconfig to find all available OpenType fonts, rather than scanning
a fixed set of directories, and getting the font-feature info from
Fontconfig rather than HarfBuzz. However, I keep getting back null
results. I see that FC_CAPABILITY is implemented, but that gives the
info in a form more designed for human readability than easy parsing.