Add optional more optimal B-frame decision method
This method (--b-adapt 2) uses a Viterbi algorithm somewhat similar to that used in trellis quantization. Note that it is not fully optimized and is very slow with large --bframes values. It also takes into account weightb, which should improve fade detection. Additionally, changes were made to cache lowres intra results for each frame to avoid recalculating them. This should improve performance in both B-frame decision methods. This can also be done for motion vectors, which will dramatically improve b-adapt 2 performance when it is complete. This patch also reads b_adapt and scenecut settings from the first pass so that the x264 header information in the output file will have correct information (since frametype decision is only done on the first pass).
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