[cairo] invalid size error, nth ping

[Paolo Bonzini]

> I actually like the simplicity!  I think I can even be tempted into liking it
> if the names are improved ("set_cacheable", ugh!).

Well, yes, {get,set}_caching_policy would really be better.  I'll
adjust my patch.

> However, first question
> is, can or can't we do that automatically?
> Cairo should just work.

Yes, we can.  I'm not sure it would not cause useless resource waste.
Maybe we can define a default policy (more than 1500 pixels => enable
caching; shamelessly stolen from Chris's xshm patch).

Thanks!

Paolo