Only predict from motion vectors that point to the same reference.
Only predict from motion vectors that point to the same reference.

This also adds median-of-two, one, and zero computation.

           RATE (%)  DSNR (dB)
    PSNR -0.33343  0.01104
 PSNRHVS -0.37274  0.01901
    SSIM -0.40101  0.01027
FASTSSIM -0.47167  0.01399

This is a win on frames with motion, but a loss on clips with no
motion and mostly illuminance motion. Taking equal_mvs into account
may be a way to avoid the loss.

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