Filter flanger
  Apply a flanging effect to the audio.
    Inputs:
       #0: default (audio)
    Outputs:
       #0: default (audio)
flanger AVOptions:
  delay             <double>     ..F.A.... base delay in milliseconds (from 0 to 30) (default 0)
  depth             <double>     ..F.A.... added swept delay in milliseconds (from 0 to 10) (default 2)
  regen             <double>     ..F.A.... percentage regeneration (delayed signal feedback) (from -95 to 95) (default 0)
  width             <double>     ..F.A.... percentage of delayed signal mixed with original (from 0 to 100) (default 71)
  speed             <double>     ..F.A.... sweeps per second (Hz) (from 0.1 to 10) (default 0.5)
  shape             <int>        ..F.A.... swept wave shape (from 0 to 1) (default sinusoidal)
     triangular      1            ..F.A....
     t               1            ..F.A....
     sinusoidal      0            ..F.A....
     s               0            ..F.A....
  phase             <double>     ..F.A.... swept wave percentage phase-shift for multi-channel (from 0 to 100) (default 25)
  interp            <int>        ..F.A.... delay-line interpolation (from 0 to 1) (default linear)
     linear          0            ..F.A....
     quadratic       1            ..F.A....


