Transformations should not have to be idempotent

[sjudd]

Currently transformations must be idempotent because we reapply them after loading thumbnails from cache. We do this so that transformations are always applied to animated gifs because we cannot cache transformed gifs.

In the short term we can solve this by moving the gif transformation into the transcoder interface. In the long run we should look in to gif encoding.

[sjudd]

Dupe of #112