Filter ssim
  Calculate the SSIM between two video streams.
    Inputs:
       #0: main (video)
       #1: reference (video)
    Outputs:
       #0: default (video)
ssim AVOptions:
  stats_file        <string>     ..FV.... Set file where to store per-frame difference information
  f                 <string>     ..FV.... Set file where to store per-frame difference information


