font/sfnt: support cmap format 6
font/sfnt: support cmap format 6

The library provides support for CMap formats 2, 4 and 12, but
does not support CMap format 6.

CL adds support for CMap format 6 (trimmed table mapping).

Change-Id: I40657c4805c14017367af17596023da96b2c7483
Reviewed-on: https://go-review.googlesource.com/c/146079
Reviewed-by: Nigel Tao <58ae08b3356928ff9c67562b4719487d54fe8348@golang.org>
Run-TryBot: Nigel Tao <58ae08b3356928ff9c67562b4719487d54fe8348@golang.org>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>

golang/image