weightp: improve scale/offset search, chroma
Rescale the scale factor if the offset clips. This makes weightp more effective in fades to/from white (and an other situation that requires big offsets). Search more than 1 scale factor and more than 1 offset, depending on --subme. Try to find the optimal chroma denominator instead of hardcoding it. Overall improvement: a few percent in fade-heavy clips, such as a sample from Avatar: TLA.
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