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
Remove unused MV visualization code
encode: do quality scaling internally
encode: factor copy_and_pad out of encode_img_in
encode: use QUALITY_SHIFT
encode: move img_dump_padded out of encode_img_in
encoder_example: remove duplicate code
logging: fix memory leak
encode: refactor
Bypass PVQ sythesis when the gain is zero
Add od_block_size enum to give names to the block sizes
Rewrite superblock splitting code to make it easier to understand

Also try to follow the coding style more closely.
Update bd_rate.m to run from any directory.