[afshaper] Delay creating `hb_set' objects until needed.

In runs on Noto Naskh Arabic, this results in 89 sets created
instead of 340 before.  Makes auto-hinter setup with HarfBuzz
enabled 20% to 30% faster.

* src/autofit/afshaper.c (af_shaper_get_coverage): Implement it.
2 files changed