Filter afir
  Apply Finite Impulse Response filter with supplied coefficients in 2nd stream.
    slice threading supported
    Inputs:
       #0: main (audio)
       #1: ir (audio)
    Outputs:
       #0: default (audio)
afir AVOptions:
  dry               <float>      ..F.A.... set dry gain (from 0 to 1) (default 1)
  wet               <float>      ..F.A.... set wet gain (from 0 to 1) (default 1)
  length            <float>      ..F.A.... set IR length (from 0 to 1) (default 1)
  again             <boolean>    ..F.A.... enable auto gain (default true)


