Reduce incorrect blank maargin on cropped blur

Author: Sergio0694

src/ImageSharp/Processing/Processors/Convolution/BokehBlurProcessor.cs

@@ -136,7 +136,7 @@ public void Invoke(int y)
 
                 // The target buffer is zeroed initially and then it accumulates the results
                 // of each partial convolution, so we don't have to clear it here as well
-                ref Vector4 targetBase = ref this.targetValues.GetElementUnsafe(0, y);
+                ref Vector4 targetBase = ref this.targetValues.GetElementUnsafe(boundsX, y);
                 ref Complex64 kernelBase = ref this.kernel[0];
 
                 for (int kY = 0; kY < kernelSize; kY++)