~30% faster smart-yuv (-pre 4) with early-out criterion
~30% faster smart-yuv (-pre 4) with early-out criterion

we look at average global improvement and stop when things are
moving slow, or when we had a quite good first iteration already
(means: the picture is "not difficult")

Change-Id: I8ab7d100353039b5b32bb5fac3fe03c8440c78d5

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