improve threshold property of gegl:unsharp-mask
So far, the new gegl-based unsharp mask filter has a very... let's say mathematical approach to parameters. As a photographer I still find the legacy filter far more familiar and easy to grasp/control.
Think of it this way: in Adobe Photoshop these parameters (amount/radius/threshold) have not changed for nearly 30 years, and they work nearly identical in Gimp's legacy filter. Why reinvent the wheel then?