Make it possible to encode robust streams with PVQ

Three changes:
1) theta is now encoded without assuming we know max_theta (most of the loss
   is from here)
2) Approximate sin(theta)=theta when computing K
3) Disable inter-band masking

Changes 1) and 2) only apply to P-frames, but 3) is even for key-frames to
avoid them looking different from P-frames.

Metrics on video-1-short:
           RATE (%)  DSNR (dB)
    PSNR 0.31829  -0.01388
 PSNRHVS 1.62467  -0.12656
    SSIM 0.86702  -0.03900
FASTSSIM 1.15187  -0.06973