Use the large-scale lookahead capability introduced in MB-tree for ratecontrol purposes. (Does not require MB-tree, however.) Greatly improved quality and compliance in 1-pass VBV mode, especially in CBR; +2db OPSNR or more in some cases. Fix some other bugs in VBV, which should improve non-lookahead mode as well. Change the tolerance algorithm in row VBV to allow for more significant mispredictions when buffer is nearly full. Note that due to the fixing of an extremely long-standing bug (>1 year), bitrates may change by nontrivial amounts in CRF without MB-tree.
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