harfbuzz: Update to 2.7.0

Overview of changes leading to 2.7.0
Saturday, July 25, 2020
====================================
- Use an implementation for round that always rounds up, some minor fluctuations
  are expected on var font specially when hb-ot callback is used.
- Fix an AAT's `kerx` issue on broken rendering of Devanagari Sangam MN.
- Remove AAT's `lcar` table support from _get_ligature_carets API, not even much
  use on macOS installed fonts (only two files).  GDEF support is the recommended
  one and expected to work properly after issues fixed two releases ago.
- Minor memory fixes to handle OOM better specially in hb-ft.
- Minor .so files versioning scheme change and remove stable/unstable scheme
  differences, was never used in practice (always default to stable scheme).
- We are now suggesting careful packaging of the library using meson,
  https://github.com/harfbuzz/harfbuzz/wiki/Notes-on-migration-to-meson
  for more information.
- Distribution package URL is changed, either use GitHub generated tarballs,
  `https://github.com/harfbuzz/harfbuzz/archive/$pkgver.tar.gz`
  or, even more preferably use commit hash of the release and git checkouts like,
  `git+https://github.com/harfbuzz/harfbuzz#commit=$commit`


(nia)