A 'superframe' is a group of frames that share the same PTS, but have a
defined decoding order. This commit adds the ability to append an index
to such a group of frames, allowing for random access to the constituent
frames. This could be useful for frame-level parallelism or partial
decoding in a multilayer scenario.

Decoding the stream serially without such an index should work as a
fallback, and VP9/TestSuperframeIndexIsOptional verifies that.

Change-Id: Idff83b7560e1a7077d8fb067bfbc45b567e78b1c