config: Increase the glyph cache size to a sensible value for parallel
painting.

The glyph cache maximum size is shared between all threads, so we need
to bump it up when we use parallel painting. Otherwise we spend 75% of
our time in cache misses on that glyph cache.
Merge pull request #36 from pcwalton/increase-cache-size

config: Increase the glyph cache size to a sensible value for parallel painting.