[cairo] glyph caching in glitz backend

[David Reveman]

I've committed some glyph caching code to the glitz backend. It seems to
be working pretty well but it still needs some more work.

The most recently used glyphs are cached in a texture, this texture is
currently always 512x512 texels but we need to make it dynamically
change size as for many applications 512x512 will be a waste of texture
memory. Glyph caches will need to be display specific, screen specific
and probably even thread specific, this is also not handled correctly
yet. I'll get these issues fixed later on.  

With a decent graphics card, glitz backend is now able to do 1M
glyphs/sec.

-David