Log Message:
harfbuzz: Update to 2.6.4

Overview of changes leading to 2.6.4
Monday, October 29, 2019
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- Small bug fix.
- Build fixes.

Overview of changes leading to 2.6.3
Monday, October 28, 2019
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- Misc small fixes, mostly to build-related issues.
- New API:
+hb_font_get_nominal_glyphs()