Filter cover_rect
  Find and cover a user specified object.
    Inputs:
       #0: default (video)
    Outputs:
       #0: default (video)
cover_rect AVOptions:
  cover             <string>     ..FV..... cover bitmap filename
  mode              <int>        ..FV..... set removal mode (from 0 to 1) (default blur)
     cover           0            ..FV..... cover area with bitmap
     blur            1            ..FV..... blur area


