Handle coefficient initialization better.

- Coefficients are no longer required to be pre-zeroed. The resulting zeroes
  are copied in even in the case where we already know they are all zeroes.

- Move cbf clearing code to only happen at the leaves of the recursion.
Clean up coefficient generation.
Avoid unnecessary copying of predicted pixels when there are no coeffs.

- These are probably from a time when reconstruction happened in this
  function.