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 <[email protected]>