font/sfnt: implement Font.GlyphName.

This is based on the post table in the sfnt file.

Change-Id: I11f7a9bd9024cfc8f92adc5abb4d5356521f0df7
Reviewed-on: https://go-review.googlesource.com/36972
Reviewed-by: David Crawshaw <crawshaw@golang.org>
4 files changed