[cairo] invalid size error, nth ping

[Paolo Bonzini]

>> 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).

Blah, I'm rusty on my own code.  Sorry.

We cannot, because if you modify the image data of a cacheable surface
 "from the outside" (touching the pixels directly), you need to call
cairo_surface_flush.

Paolo