Remove inter-band masking

It never helped much (and now it's hurting curves slightly apparently) and
it blocks doing entropy decoding independently from (and in parallel with)
PVQ. Also, we had to disable it for robust streams anyway.

subset1 results:
           RATE (%) DSNR (dB)
    PSNR -0.745795 0.0404235
 PSNRHVS -0.263827 0.017698
    SSIM -0.42999 0.0204819
FASTSSIM -0.113949 0.00366062
Makes keyframes entropy decoding independent of PVQ

This makes it possible to do the intropy decoding in parallel with the PVQ
decoding.
subset1 results:
           RATE (%) DSNR (dB)
    PSNR 1.97396 -0.110506
 PSNRHVS 1.77392 -0.124211
    SSIM 2.05182 -0.0904858
FASTSSIM 1.68843 -0.0548173
Replace noref=1,gain=0 by a skip on inter when the reference is small

This makes us use noref much less often.
video1-short results:
           RATE (%)  DSNR (dB)
    PSNR -1.68026  0.08641
 PSNRHVS -2.10895  0.15816
    SSIM -1.25716  0.05708
FASTSSIM -3.63240  0.22229