Log Message:
harfbuzz: Update to 2.6.8

Overview of changes leading to 2.6.8
Monday, June 22, 2020
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- New API to fetch glyph alternates from GSUB table.
- hb-coretext build fix for macOS < 10.10.
- Meson build fixes, cmake port removal is postponed but please prepare for
  it and give us feedback.
  Autotools is still our main build system however please consider
  experimenting with meson also for packaging the library.
- New API:
+hb_ot_layout_lookup_get_glyph_alternates()