Selective Gaussian Blur Aux Input

[Ofnuts]

From what I understand of the source code:

• Selective blur is a Gaussian blur that is applied only in low-contrast areas,  i.e., in areas where the difference between a pixels and its neighboring pixels is under some threshold.
  
• In normal operation the contrast is computed on the layer that is blurred
  
• When an aux input layer is provided, the contrast is computed on that layer instead of being computed on the blurred layer