font: add bounds measurement methods
font: add bounds measurement methods

Fixes golang/go#18297

Change-Id: Ib873b18c9bc5cac0d820c6129d9a28cb44f2a8da
Reviewed-on: https://go-review.googlesource.com/34514
Reviewed-by: Nigel Tao <58ae08b3356928ff9c67562b4719487d54fe8348@golang.org>

golang/image