font/basicfont: add Descent and Left fields.

A face's glyph height (the height of the glyph image) isn't necessarily
the inter-line spacing: f.Ascent + f.Descent isn't always f.Height.

Similarly, a face's glyph image's left side isn't always dot.X.

Change-Id: I86594c52b8d31bf652ac34a695e9270ac635a5bc
Reviewed-on: https://go-review.googlesource.com/24481
Reviewed-by: David Crawshaw <crawshaw@golang.org>
1 file changed