checkasm: Add functions for printing pixel buffers

When developing assembly functions, some easy printing of the failure cause is very helpful. (So far, I've always written something like this for all implemented functions, but not really upstreamed it so far.) IIRC the x264 checkasm has got some visual printing of test results like this.

This adds the general framework to checkasm, and uses it for the itx tests as an example. Printouts for other tests can be filled in as need arises.