Defining skip as theta=-1 for keyframes since it's much more common

subset1:
           RATE (%) DSNR (dB)
    PSNR -0.375649 0.0213739
 PSNRHVS -0.433576 0.0292169
    SSIM -0.3784 0.0173524
FASTSSIM -0.446371 0.0144125

video1-short:
           RATE (%)  DSNR (dB)
    PSNR -0.01509  0.00077
 PSNRHVS -0.01745  0.00129
    SSIM -0.01507  0.00068
FASTSSIM -0.01611  0.00092
fixes for issue 512