2-pass VBV fixes
Properly run slicetype frame cost with 2pass + MB-tree. Slash the VBV rate tolerance in 2-pass mode; increasing it made sense for the highly reactive 1-pass VBV algorithm, but not for 2-pass. 2-pass's planned frame sizes are guaranteed to be reasonable, since they are based on a real first pass, while 1-pass's, based on lookahead SATD, cannot always be trusted.
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