[cairo] Getting cairo to clean up after itself

[Bill Spitzak]

Keith Packard wrote:

> An assertion that checked that each hash table really was empty would be
> useful here. That the function is called cairo_debug should steer normal
> applications away from trying to use it while things were active.

Actually I think it would be ok if Cairo does not work after using this 
function. It's only purpose is to call it just before exit when doing a 
valgrind. It frees everything Cairo knows about so that actual leaked 
data can be detected. But attempting to call Cairo again is a programmer 
error and should crash, so nobody thinks it's a good idea to put this in 
the middle of the program.