Filter blackframe
  Detect frames that are (almost) black.
    Inputs:
       #0: default (video)
    Outputs:
       #0: default (video)
blackframe AVOptions:
  amount            <int>        ..FV..... percentage of the pixels that have to be below the threshold for the frame to be considered black (from 0 to 100) (default 98)
  threshold         <int>        ..FV..... threshold below which a pixel value is considered black (from 0 to 255) (default 32)
  thresh            <int>        ..FV..... threshold below which a pixel value is considered black (from 0 to 255) (default 32)


