[cairo] text measurement in perf?

[Baz]

Turs out we are holding the mutex everywhere needed for this
optimisation to work. Test and patch available at:
http://gitweb.freedesktop.org/?p=users/brianewins/cairo;a=shortlog;h=patches/glyph-cache

The test manages to get uncached metrics for ATSUI, FT, havent tried
it on windows. I see around 1.7x speedup fetching uncached metrics
with this patch, with both font systems. Obviously this won't make
much difference to simple apps with western languages only, since
cache misses will be rare; but it will help displaying non-western
scripts. Comments welcome.

The penalty for a cache miss still seems incredibly high though - it
may well be worth trying a warm cache, just cache entries for the
first 255 glyph ids with no metrics attached, though the setup cost
might not be worth it.