[HarfBuzz] Transparent, lock-free, shape plan caching

[Behdad Esfahbod]

Hi,

I just pushed out the results of my lone night hacking last night,
implementing universal shape planning and transparent caching of shape plans.
Caching is done only when there are no user-selected font features.

What this means, for users of the library, simply is a measurable performance
boost specially with short strings and lookup-intensive (cursive, Indic, ...)
fonts.

I don't quite like the macro-intensive implementation right now, and will
probably rewrite it over the weekend.

There's a minor API implication: users now have to call hb_ot_layout_ensure()
before using any of hb-ot-layout.h API.  But that's for advanced users only...

Would be nice to see numbers from Firefox / Chrome.  This should be more of a
win for Firefox since it shapes word at a time.

Cheers,
behdad