Filter subtitles
  Render text subtitles onto input video using the libass library.
    Inputs:
       #0: default (video)
    Outputs:
       #0: default (video)
subtitles 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)
  charenc           <string>     ..FV.... set input character encoding
  stream_index      <int>        ..FV.... set stream index (from -1 to INT_MAX) (default -1)
  si                <int>        ..FV.... set stream index (from -1 to INT_MAX) (default -1)
  force_style       <string>     ..FV.... force subtitle style


