Refactor write_modes_b() and decode_block()

In order to reduce the code complexity for handling parameter
coding and recon separately for each 64x64 in non-causal obmc
experiment, we break them down to two steps calling separate
functions, one for params, the other dealing with coefficients
and recon(decoder side).
Note: actually the non-causal prediction can use the original
syntax, but right now in the decoder coeff detoken and recon are
heavily nested.

Change-Id: I72d9c42ab8f38b57850d6b0481551893f1702822
2 files changed