More accurate MV rate estimation

This could be even more accurate by making mv_small_rate_est adaptive or
by using a better estimate for the >=3 part of the component magnitudes,
but so far my attempts to do so have only decreased quality.

Results on video-1-short:
           RATE (%)  DSNR (dB)
    PSNR -0.89100  0.04614
 PSNRHVS -1.04905  0.08080
    SSIM -1.09365  0.05120
FASTSSIM -1.30466  0.07874

Results on sintel_trailer_2k_short:
           RATE (%)  DSNR (dB)
    PSNR -4.66465  0.22110
 PSNRHVS -5.80139  0.31411
    SSIM -4.75017  0.16922
FASTSSIM -4.92864  0.21090