Log Message:
Updated harfbuzz to 1.4.8.

Overview of changes leading to 1.4.8
Tuesday, August 8, 2017
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- Major fix to avar table handling.
- Rename hb-shape --show-message to --trace.
- Build fixes.

Overview of changes leading to 1.4.7
Tuesday, July 18, 2017
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- Multiple Indic, Tibetan, and Cham fixes.
- CoreText: Allow disabling kerning.
- Adjust Arabic feature order again.
- Misc build fixes.