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