bidir: fix for context decision for mv block split pattern. For 2 B-frames, this gives rate change of:            RATE (%)  DSNR (dB)     PSNR -0.00508  0.00015  PSNRHVS 0.00074  -0.00003     SSIM -0.00176  0.00004 FASTSSIM 0.00272  -0.00007
bidir: fix for context decision for mv block split pattern.
For 2 B-frames, this gives rate change of:
           RATE (%)  DSNR (dB)
    PSNR -0.00508  0.00015
 PSNRHVS 0.00074  -0.00003
    SSIM -0.00176  0.00004
FASTSSIM 0.00272  -0.00007

HeadhunterXamd/daala,xiphmont/daala,HeadhunterXamd/daala,jmvalin/daala,KyleSiefring/daala,luctrudeau/daala,luctrudeau/daala,HeadhunterXamd/daala,xiphmont/daala,HeadhunterXamd/daala,xiph/daala,xiph/daala,jmvalin/daala,xiphmont/daala,HeadhunterXamd/daala,luctrudeau/daala,xiphmont/daala,ycho/daala,ycho/daala,jmvalin/daala,xiph/daala,jmvalin/daala,luctrudeau/daala,KyleSiefring/daala,KyleSiefring/daala,ycho/daala,xiph/daala,KyleSiefring/daala,ycho/daala,KyleSiefring/daala,xiph/daala,xiphmont/daala,luctrudeau/daala,ycho/daala,jmvalin/daala