[cairo] [patch] caching, glyph, font, and glyphset patch, round 2

[graydon hoare]

Keith Packard wrote:

> If most rendering is going to the screen, cairo can discard the actual 
> pixel data after sending it to the X server which *does* share the images 
> among multiple applications.  I'm pretty sure the current cache code 
> doesn't do that yet.

no, but it's plausible. there are a couple ways to do it: either free 
and null out the image field inside a image+metrics cache entry when you 
upload it (and make the lookup function for the image+metrics cache 
rebuild it on demand), or split the image+metrics cache into two 
separate caches and flush the image entries. the latter approach is 
probably a bit cleaner, but might use more memory. or might not. it's 
hard to guess. both would work, anyways.

-graydon