pvq: convert compute_max_theta to fixed (aside from beta)
pvq: convert compute_max_theta to fixed (aside from beta)

ntt-short1:
PSNR YCbCr:      0.05%      0.61%      0.34%
   PSNRHVS:      0.06%
      SSIM:      0.01%
    MSSSIM:      0.02%
 CIEDE2000:      0.14%

subset1:
           LOW (%)  MEDIUM (%) HIGH (%)
    PSNR  0.002409  0.001289  0.014176
 PSNRHVS  0.002905 -0.000071  0.003998
    SSIM -0.000910  0.000938  0.001277
FASTSSIM -0.004575 -0.008021 -0.001923

ycho/daala,xiph/daala,xiph/daala,ycho/daala,ycho/daala,ycho/daala,ycho/daala,luctrudeau/daala,luctrudeau/daala,xiph/daala,luctrudeau/daala,luctrudeau/daala,luctrudeau/daala,xiph/daala,xiph/daala