Add a new API, hb_ot_layout_get_feature_name_ids (#976)

This new API returns cvXX and ssXX related NameId, things like
featUiLabelNameId, featUiTooltipTextNameId, sampleTextNameId, ... of cvXX
and UINameId of ssXX, in a unified way.

However HarfBuzz currently doesn't expose an API for retrieving the actual
information associated with NameId from the `name` table and that should be
done separately.
4 files changed