HarfBuzz shaping support

Yeah, it's pretty much working now... after hunting down a few of stupid bugs. libass can now render text without all kinds of OpenType features. Here are a few samples.

Arabic text with diacritics (buggy with FriBidi's shaper).

Connected handwriting (using the Zaner-Bloser Schoolhouse font)

Automatic fractions (using the Calluna font)