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