WIP: Correct default CbCrQpOffset from -1 to -2 after integration of JVET-O0650

This is related to the combination of joint coding of chroma residuals (JCCR) and the recent integration of (if I'm not mistaken) JVET_O0650_SIGNAL_CHROMAQP_MAPPING_TABLE. Quoting Frank's reply to the email "Chroma QP tables for CTC" from Friday, August 9:

it turns out that I baked in the +1 offset from CbQpOffset and CrQpOffset (see VTM 5 config) into the mapping table. The CbCrQpOffset value would need to be changed from its default -1 value to -2 to compensate for the +1 included into the table and have PSNR values match.

This merge request does as suggested and, by doing so, recovers the BD-rate gain of the JCCR tool, as it was before the integration of JVET_O0650_SIGNAL_CHROMAQP_MAPPING_TABLE. An alternative solution would be to set

CbCrQpOffset : -2

in the encoder cfg files, but I prefer this approach. Any comments?

Christian