font: add a MeasureString function, not just a method.

It can be useful to measure some text without having to set up a Drawer.

Change-Id: I18b7b1fecc32ca69b7644d598ed21462e7c41edd
Reviewed-on: https://go-review.googlesource.com/21785
Reviewed-by: Andrew Gerrand <adg@golang.org>
Reviewed-by: David Crawshaw <crawshaw@golang.org>
1 file changed