Filter elbg
  Apply posterize effect, using the ELBG algorithm.
    Inputs:
       #0: default (video)
    Outputs:
       #0: default (video)
elbg AVOptions:
  codebook_length   <int>        ..FV..... set codebook length (from 1 to INT_MAX) (default 256)
  l                 <int>        ..FV..... set codebook length (from 1 to INT_MAX) (default 256)
  nb_steps          <int>        ..FV..... set max number of steps used to compute the mapping (from 1 to INT_MAX) (default 1)
  n                 <int>        ..FV..... set max number of steps used to compute the mapping (from 1 to INT_MAX) (default 1)
  seed              <int>        ..FV..... set the random seed (from -1 to UINT32_MAX) (default -1)
  s                 <int>        ..FV..... set the random seed (from -1 to UINT32_MAX) (default -1)
  pal8              <boolean>    ..FV..... set the pal8 output (default false)


