font/sfnt: add a ppem arg to Font.LoadGlyph.

This lets us load a glyph at e.g. 12 pixels per em.

Change-Id: I048b3db89af8670782953a8361afe0e6373df9b0
Reviewed-on: https://go-review.googlesource.com/37175
Reviewed-by: David Crawshaw <crawshaw@golang.org>
6 files changed