Added gradient kernel

SDFMapCreator/SdfKernels.Kernels.cs

@@ -69,5 +69,15 @@ public partial class SdfKernels {
        
         sdf[index] = new(minDist);
     }
-    
-}
+
+    private static void GradientKernel(Index2D index,
+        ArrayView2D<Vector3, Stride2D.DenseX> mask,
+        ArrayView2D<Vector3, Stride2D.DenseX> sdfa,
+        ArrayView2D<Vector3, Stride2D.DenseX> sdfb,
+        ArrayView2D<Vector3, Stride2D.DenseX> gradient
+    ) { //early exit if not on mask
+        if(mask[index].X == 0f || mask[index].Y == 0f) return;
+        float a = sdfa[index].X; float b = sdfb[index].X;
+        gradient[index] = new (a / (a + b));
+    }
+}