title: GPUImage CannyEdgeDetectionFilter with white edges?
body: <p>I'm using GPUImage from the Git repository to apply some filters to images. I notice that the edge detection filters (I'm using the CannyEdgeDetectionFilter specifically) paint white edges on black background. Anyone know how to make that black edges on white background - or another filter that would do it (and not be heavyweight since that's the only filter I would need from that library).</p>
  
  <p>Thanks
  Tony.</p>