Filter palettegen
  Find the optimal palette for a given stream.
    Inputs:
       #0: default (video)
    Outputs:
       #0: default (video)
palettegen AVOptions:
  max_colors        <int>        ..FV..... set the maximum number of colors to use in the palette (from 4 to 256) (default 256)
  reserve_transparent <boolean>    ..FV..... reserve a palette entry for transparency (default true)
  transparency_color <color>      ..FV..... set a background color for transparency (default "lime")
  stats_mode        <int>        ..FV..... set statistics mode (from 0 to 2) (default full)
     full            0            ..FV..... compute full frame histograms
     diff            1            ..FV..... compute histograms only for the part that differs from previous frame
     single          2            ..FV..... compute new histogram for each frame


