When rate control is enabled, enable cu_qp_delta_enabled_flag in PPS
with diff_cu_qp_delta_depth set to 0. Also adds code for writing the QP
deltas and a new cabac context.
Write bitstream without chroma when encoding with --input-format=P400.
This reduces bitstream size by 0-1 %, compared to coding monochrome in
420 format, and speeds up encoding slightly due to not processing
chroma.
