Special case for encoding K=1 PVQ codewords

Subset1:
           RATE (%) DSNR (dB)
    PSNR -0.13131 0.00814259
 PSNRHVS -0.184684 0.0125804
    SSIM -0.139072 0.00717853
FASTSSIM -0.225148 0.00738782

Video1-short:
           RATE (%)  DSNR (dB)
    PSNR -0.04805  0.00251
 PSNRHVS -0.07723  0.00581
    SSIM -0.04611  0.00214
FASTSSIM -0.07197  0.00417