[cairo] cacheable image representation now on a branch

[Paolo Bonzini]

> But the large-source test only uses a 20x20 source - surely Quartz is
> not failing to allocate that! Oh I think I see the issue - it attempts
> to make a CGImage large enough to accommodate the whole image and not
> just cover the region of interest - shame on Quartz for having such
> small image surfaces! (However CAIRO_STATUS_SURFACE_TOO_BIG sounds like
> a good idea and would clarify a few errors which are currently
> mis-reported as NO_MEMORY.)

I pushed it to invalid-size-error in my repository and rebased the cache
branch on top of it.

I decided to leave large-source as a failure on Quartz, it must not be
too hard to fix it for real.

Paolo