“Beauty, it is often said, is in the eye of the beholder, and so might be copyright infringement. Artist Richard Prince never denied that he made use of some photographic images he found in a 2000 book by Patrick Cariou…. The question,” central to Cariou’s lawsuit against Prince, “is whether Mr. Prince’s use of these images was ‘transformative’ — borrowing in the process of creating something entirely new — or just stealing.”