Much more accurate B-skip detection at 2 < subme < 7
Use the same method that x264 uses for P-skip detection. This significantly improves quality (1-6%), but at a significant speed cost as well (5-20%). It also may have a very positive visual effect in cases where the inaccurate skip detection resulted in slightly-off vectors in B-frames. This could cause slight blurring or non-smooth motion in low-complexity frames at high quantizers. Not all instances of this problem are solved: the only universal solution is non-locally-optimal mode decision, which x264 does not currently have. subme >= 7 or <= 2 are unaffected.
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