pvq: use integer approximation for exp2
pvq: use integer approximation for exp2

ntt-short1:
PSNR YCbCr:      0.06%      0.28%      0.23%
   PSNRHVS:      0.05%
      SSIM:      0.04%
    MSSSIM:     -0.00%
 CIEDE2000:     -0.01%

subset1:
          LOW (%)  MEDIUM (%) HIGH (%)
    PSNR 0.016847 -0.010997  0.010651
 PSNRHVS 0.010134 -0.007417 -0.019350
    SSIM 0.014395 -0.016214 -0.010879
FASTSSIM 0.045682 -0.004407 -0.045149

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