This was actually more of a feathering feature I added earlier, and we
already have a function for that: gimp_gegl_apply_feather(). This is
using a gaussian blur, just as what I was doing anyway. This commit also
adds the "Feather Radius" scale, similar to other tools with the
"Feather Edges". So that makes it consistent (and more useful as you can
adapt to your needs).