Filter mix
  Mix video inputs.
    slice threading supported
    Inputs:
        dynamic (depending on the options)
    Outputs:
       #0: default (video)
mix AVOptions:
  inputs            <int>        ..FV..... set number of inputs (from 2 to INT_MAX) (default 2)
  weights           <string>     ..FV..... set weight for each input (default "1 1")
  scale             <float>      ..FV..... set scale (from 0 to 32767) (default 0)
  duration          <int>        ..FV..... how to determine end of stream (from 0 to 2) (default longest)
     longest         0            ..FV..... Duration of longest input
     shortest        1            ..FV..... Duration of shortest input
     first           2            ..FV..... Duration of first input


