Log Message:
harfbuzz: update to 2.6.0.

Overview of changes leading to 2.6.0
Tuesday, August 13, 2019
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- New OpenType metrics, baseline, and metadata table access APIs.
- New API to set font variations to a named-instance.
- New hb-gdi.h header and API for creating hb_face_t from HFONT.
- Amalgam: Provide a single-file harfbuzz.cc file for easier alternate building.
- More size-reduction configurable options, enabled by HB_TINY.
- New API:
+hb_font_set_var_named_instance()
+hb_gdi_face_create()
+hb_ot_layout_baseline_tag_t
+hb_ot_layout_get_baseline()
+hb_ot_meta_tag_t
+hb_ot_meta_get_entry_tags()
+hb_ot_meta_reference_entry()
+hb_ot_metrics_tag_t
+hb_ot_metrics_get_position()
+hb_ot_metrics_get_variation()
+hb_ot_metrics_get_x_variation()
+hb_ot_metrics_get_y_variation()