EC_ADAPT: Perform backwards updates directly on CDFs.
The initial CDF for each frame is stored in the frame context. CDFs for actual coding are stored in the tile structures, and these are what get adapted. The initial CDF is replaced by an average CDF derived from these tile CDFs. This is carried forward to future frames when backward adaptation is on. CDFs are no longer set from the 8 bit probabilities in backwards adaptation. For now, 8 bit probabilities are maintained for use in the encoder and for symbols which do not have a CDF. Change-Id: I106b30510bfad1fa57d077f7702acc1864378a09