Filter pad
  Pad the input video.
    Inputs:
       #0: default (video)
    Outputs:
       #0: default (video)
pad AVOptions:
  width             <string>     ..FV..... set the pad area width expression (default "iw")
  w                 <string>     ..FV..... set the pad area width expression (default "iw")
  height            <string>     ..FV..... set the pad area height expression (default "ih")
  h                 <string>     ..FV..... set the pad area height expression (default "ih")
  x                 <string>     ..FV..... set the x offset expression for the input image position (default "0")
  y                 <string>     ..FV..... set the y offset expression for the input image position (default "0")
  color             <color>      ..FV..... set the color of the padded area border (default "black")
  eval              <int>        ..FV..... specify when to evaluate expressions (from 0 to 1) (default init)
     init            0            ..FV..... eval expressions once during initialization
     frame           1            ..FV..... eval expressions during initialization and per-frame
  aspect            <rational>   ..FV..... pad to fit an aspect instead of a resolution (from 0 to DBL_MAX) (default 0/1)


