Stop re-applying transformations when decoding from cache

[sjudd]

We currently do so to make sure transformations are applied to GIFs. We can't cache transformed GIFs because we lack a gif encoder. A better workaround than reapplying transformations would be instead to use a transcoder. Even better would be to find and add some kind of gif encoder.

[sjudd]

Added a GifEncoder here: f7b3e5d. It's now no longer a requirement to transform cached images.