Various 1-pass VBV tweaks
Make predictors have an offset in addition to a multiplier. This primarily fixes issues in sources with lots of extremely static scenes, such as anime and CGI. We tried linear regressions, but they were very unreliable as predictors. Also allow VBV to be slightly more aggressive in raising QPs to avoid not having enough bits left in some situations. Up to 1db improvement on some clips.
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