PVQ refactor: moves all rate heuristics to od_pvq_rate()

No change in actual behaviour
Dumping the number of bits per symbol from the entropy coder

Let's maximize that.
Using actual k-tokenizer rate in PVQ RDO

subset1:
           RATE (%) DSNR (dB)
    PSNR -0.362096 0.0227787
 PSNRHVS -0.767835 0.0560247
    SSIM -0.495929 0.0216287
FASTSSIM -0.369065 0.012262

video1-short:
           RATE (%)  DSNR (dB)
    PSNR -1.12440  0.05763
 PSNRHVS -0.33475  0.02480
    SSIM -1.28665  0.05854
FASTSSIM -0.36200  0.02063