code: @Override
      public ImageSource apply(ImageSource source) {
          if (radius != 0) {
              if (source.isGrayscale()) {
                  return applyGrayscale(source, radius);
              } else {
                  return applyRGB(source, radius);
              }
          } else {
              if (source.isGrayscale()) {
                  return applyGrayscale(source, kernel);
              } else {
                  return applyRGB(source, kernel);
              }
          }
      }
docs: Apply filter to an image.
  
  @param source FastBitmap