Filter acrossover
  Split audio into per-bands streams.
    slice threading supported
    Inputs:
       #0: default (audio)
    Outputs:
        dynamic (depending on the options)
acrossover AVOptions:
  split             <string>     ..F.A...... set split frequencies (default "500")
  order             <int>        ..F.A...... set filter order (from 0 to 9) (default 4th)
     2nd             0            ..F.A...... 2nd order (12 dB/8ve)
     4th             1            ..F.A...... 4th order (24 dB/8ve)
     6th             2            ..F.A...... 6th order (36 dB/8ve)
     8th             3            ..F.A...... 8th order (48 dB/8ve)
     10th            4            ..F.A...... 10th order (60 dB/8ve)
     12th            5            ..F.A...... 12th order (72 dB/8ve)
     14th            6            ..F.A...... 14th order (84 dB/8ve)
     16th            7            ..F.A...... 16th order (96 dB/8ve)
     18th            8            ..F.A...... 18th order (108 dB/8ve)
     20th            9            ..F.A...... 20th order (120 dB/8ve)
  level             <float>      ..F.A...... set input gain (from 0 to 1) (default 1)
  gain              <string>     ..F.A...... set output bands gain (default "1.f")


