Edit's for Advanced Graphics Chapter

Hey @arturo, here are some edits for the Advanced Graphics Chapter. I did everything inline, with just a few things to mention:

- How about adding a brief section on `ofNode`. Its a very useful and important 3D class, and one that gets mentioned multiple times throughout the chapter.
- With a more updated feeling for the chapters that will, and will not, be included in ofBook (at least at first), it might be worth giving a few concepts a bit more attention.

I've been enjoying reading your chapters a lot :+1: . I've got my eye on the Memory chapter next.