[Fontconfig] Speeding up caching

[Behdad Esfahbod]

See discussion here:

  https://bugs.freedesktop.org/show_bug.cgi?id=64766

In particular, I'm suggesting that we remove FC_HASH, and make changes to how
we check glyphs for outline.  I measure something like 30x speedup.

I'm faulty for suggesting that we push FC_HASH in without really measuring the
impact.  I was under the impression that computing the digest will not take a
significant time compared to all the other stuff we do.  I was wrong.

Akira, what do you think?  I can implement the 'loca' table stuff.

-- 
behdad
http://behdad.org/