Filter ass
  Render ASS subtitles onto input video using the libass library.
    Inputs:
       #0: default (video)
    Outputs:
       #0: default (video)
ass AVOptions:
  filename          <string>     ..FV..... set the filename of file to read
  f                 <string>     ..FV..... set the filename of file to read
  original_size     <image_size> ..FV..... set the size of the original video (used to scale fonts)
  fontsdir          <string>     ..FV..... set the directory containing the fonts to read
  alpha             <boolean>    ..FV..... enable processing of alpha channel (default false)
  shaping           <int>        ..FV..... set shaping engine (from -1 to 1) (default auto)
     auto                         ..FV.....
     simple                       ..FV..... simple shaping
     complex                      ..FV..... complex shaping


