fix: segmentation fault --tune zerolatency
rc: rename variable vbvMinRate to isCbr
rc: always use frameSizeEstimted for bits in updateVbvPlan
psyrd: is enabled only at rdLevels 5 and 6.

Only rdLevels 5 and 6 use full-RDO based decisions. These are used in presets slower, veryslow and placebo.
rest: refine rdLevels description.