Filter afir
  Apply Finite Impulse Response filter with supplied coefficients in 2nd stream.
    slice threading supported
    Inputs:
       #0: main (audio)
       #1: ir (audio)
    Outputs:
        dynamic (depending on the options)
afir AVOptions:
  dry               <float>      ..F.A.... set dry gain (from 0 to 10) (default 1)
  wet               <float>      ..F.A.... set wet gain (from 0 to 10) (default 1)
  length            <float>      ..F.A.... set IR length (from 0 to 1) (default 1)
  again             <boolean>    ..F.A.... enable auto gain (default true)
  maxir             <float>      ..F.A.... set max IR length (from 0.1 to 60) (default 30)
  response          <boolean>    ..FV..... show IR frequency response (default false)
  channel           <int>        ..FV..... set IR channel to display frequency response (from 0 to 1024) (default 0)
  size              <image_size> ..FV..... set video size (default "hd720")


