Expose ascent and descent in truetype.Font.

[GoogleCodeExporter]

These are needed to figure out line height and the height for the background of 
highlighted text. Font.Bounds().YMin and YMax don't work; they are too large. 
Instead we need ascent and descent from the hhea table.

Original issue reported on code.google.com by burns.ethan@gmail.com on 16 Feb 2015 at 8:13

[stephenwithav]

I second this notion.

[stephenwithav]

Mind if I tackle this one in the near future, @nigeltao?

[nigeltao]

Sure, have at it, although there might be implications for what we want to do for the x/image/font package wrt exposing ascent / descent / height in a uniform way across all font formats, not just the truetype font format. That's the "TODO: per-font Metrics" in https://go.googlesource.com/image/+/master/font/font.go

[stephenwithav]

Has there been any/much discussion on how to expose them that I can look at before starting?

[nigeltao]

No, no discussion yet.

[tv42]

Maybe deprecate this code in favor of https://godoc.org/golang.org/x/image/font#Metrics ?
I was able to successfully render text with just x/image/font, so this library seems to no longer be needed.