Changes function kvz_get_coeff_cost to only copy the CABAC contexts and
not the whole encoder state.
Other threads could be simultaneously using the other parts of the
encoder state. Only copying the CABAC fixes a TSan data race warning.
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.
