This CL adds the type Face that implements the font.Face interface.
This CL also adds tests using gofont/goregular as an input font, using
github.com/golang/freetype/truetype as reference values.
Updates golang/go#22451.
Change-Id: I2a6945309331b251ec2ddec95b6e809ad10aa116
Reviewed-on: https://go-review.googlesource.com/73870
Reviewed-by: Nigel Tao <nigeltao@golang.org>
