Bug 1726416 - Update HarfBuzz to 2.9.1. r=jfkthame

gfx/harfbuzz/COPYING

gfx/harfbuzz/Makefile.am

gfx/harfbuzz/NEWS

gfx/harfbuzz/README-mozilla

gfx/harfbuzz/README.md

gfx/harfbuzz/configure.ac

gfx/harfbuzz/src/Makefile.am

gfx/harfbuzz/src/Makefile.sources

gfx/harfbuzz/src/check-libstdc++.py

gfx/harfbuzz/src/check-static-inits.py

gfx/harfbuzz/src/check-symbols.py

gfx/harfbuzz/src/gen-emoji-table.py

gfx/harfbuzz/src/gen-ragel-artifacts.py

gfx/harfbuzz/src/gen-use-table.py

gfx/harfbuzz/src/harfbuzz.cc

gfx/harfbuzz/src/hb-aat-layout.cc

gfx/harfbuzz/src/hb-algs.hh

gfx/harfbuzz/src/hb-array.hh

gfx/harfbuzz/src/hb-bimap.hh

gfx/harfbuzz/src/hb-bit-page.hh