Comment 6 for bug 217908

To summarize that comment, I think it makes sense to have a preference for maximal size of the pixmap cache we end up with. Once we get there, we stop decoding new images that would take us over the cache size. The default can be either infinite or something large enough that not rendering images is better than trying to render them at that point. Deployments in low-resource situations (e.g. mobiles with no virtual memory, thin clients, etc) can set the preference as needed.