Add variable sign to the ab_compare*.sh scripts

Signed-off-by: Nathan E. Egge <negge@dgql.org>
Removes code duplication in block size decoder
Using od_encode_cdf_adapt() for the 4x4 vs 8x8 decision too

Removes useless training code
Adds per-plane context for the PVQ gain.

subset1:
           RATE (%) DSNR (dB)
    PSNR -0.905569 0.0510298
 PSNRHVS -1.00289 0.0719411
    SSIM -0.898377 0.0399292
FASTSSIM -0.931303 0.0302713

Helps a bit on parkjoy, hurts a bit on akiyo (because it's small)
dct: use OD_MOVE

This avoids a bug when building with clang-3.4