No inheritance of computed-files

[masiulaniec]

An image stream may have a filter.add and a triggers.add but not computed-files.add. This causes a tremendous amount of duplication.

[rgooch]

Interesting. We haven't stacked images with computed files so we haven't had this need, but it makes sense. Please let me know if you're planning on sending a PR, otherwise poke me in a week or two and I'll cook something up :-)