[cairo] Global glyph cache

[Jeff Muizelaar]

It looks like the new global glyph cache is responsible for a 10MB
increase in memory usage on a firefox tp run. This increase is keeping
me from merging cairo trunk into mozilla-central.

Looking through the code this seems expected. Given a cache page size of
17936 bytes we can get up 512*17936 = 9183232 bytes in the cache. In
addition, we could have glyph pages attached to scaled_fonts which would
add another 256 pages bringing the maximum up to 13795584 or 13.1 MB. 

It also looks like some of the cache freezing/thawing might be broken
because even after adjusting MAX_GLYPH_PAGES_CACHED to 2 I've still seen
around 400 active glyph pages during a tp run.

-Jeff