It's a hb_face_t that has add_table() and in the future knows how to compile itself into a font blob.
6804b61d [subset] Add hb_subset_face
src/hb-private.hh | 9 ++++++
src/hb-subset.cc | 88 +++++++++++++++++++++++++++++++++++++++++++++++++++++++
2 files changed, 97 insertions(+)
Upstream: cgit.freedesktop.org