Log Message:
harfbuzz: update to 1.7.6.

Overview of changes leading to 1.7.6
Wednesday, March 7, 2018
====================================

- Fix to hb_set_t binary operations. Ouch.
- New experimental harfbuzz-subset library. All of hb-subset.h
  is experimental right now and API WILL change.

- New API:
hb_blob_copy_writable_or_fail()
HB_OT_TAG_BASE
hb_set_previous()
hb_set_previous_range()