title: ffmpeg delogo works not good when delogo at boundary
body: <p>I am trying using ffmpeg delogo filter to hide the logo, but I found when the logo is appears at boundary, delogo filter seems works not good. please check following images.</p>
  
  <p><a href="https://i.stack.imgur.com/OwMZe.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/OwMZe.png" alt="enter image description here"></a>
  <a href="https://i.stack.imgur.com/HTbfv.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/HTbfv.png" alt="enter image description here"></a></p>
  
  <p>I also read a bit of source, seems the algorithm need subtract 1 pixel boundary. in the second image, the delogo result is looks strange, bottom is white. I want to make it invisible, is it possible?</p>
  
  <p>thanks</p>