Merge pull request #36 from stianse/fix-conformance

Fix cu_qp_delta_abs
fix qp prediction and store

- Fix QP prediction logic for checking if neighbor is in same Ctb.
- Set QP for entire CU, not just the MinCuQPDeltaSize, for using
  correct QP during deblocking.
- Only call decode_quantization_parameters() once per CU, or twice if
  qp delta is enabled.
fix warnings about unused variables and functions