This new feature gives the user more control over *when* their images
are validated. Image cache backends are now exclusively for controlling
the *how*. This means you won't have to write a lot of code when you
just want to change one or the other.
While this change means users can no longer specify their own filenames,
changing a property of a processor, for example, will now result in a
new image. This solves a lot of the previous invalidation issues.
