font/sfnt: support for kerning from GPOS tables

This CL adds support for reading kerning information from GPOS tables.

Most modern fonts like Noto or Roboto do not provide a kern table.
Instead they use the GPOS table for kerning.

This CL is limited to horizontal kerning and latin scripts.

The proprietary_test was extended to check all supported variations.
gpos.go has full test coverage (excluding error conditions).
Additionally, the new TestBulkKern (opt-in) can be used to check parsing
of large font collection.

Fixes: golang/go#29528

Change-Id: I9f1ae142c9e26170dd836ccaca1526bbefe9698a
Reviewed-on: https://go-review.googlesource.com/c/159638
Reviewed-by: Nigel Tao <nigeltao@golang.org>
4 files changed
tree: 59c9696ff00c4166aa27cd0dead70129d95240a9
  1. .gitattributes
  2. .gitignore
  3. AUTHORS
  4. CONTRIBUTING.md
  5. CONTRIBUTORS
  6. LICENSE
  7. PATENTS
  8. README.md
  9. bmp/
  10. cmd/
  11. codereview.cfg
  12. colornames/
  13. draw/
  14. example/
  15. font/
  16. math/
  17. riff/
  18. testdata/
  19. tiff/
  20. vector/
  21. vp8/
  22. vp8l/
  23. webp/
README.md

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