This allows transformations to be applied that
for whatever reason cannot be cached. In 
particular this is required for transformations
for GIFs because neither Glide nor the Android
framework include a GIF decoder so we can only
cache the original data. This change is risky,
and relies on transformations being idempotent.

Conflicts:
	library/src/test/java/com/bumptech/glide/load/engine/EngineJobTest.java