Fix dynamicItemOpacity not work on last item when anchor MIDDLE

[MarnoDev]

  double calculateOpacity(int index) {
    //scroll-pixel position for index to be at the center of ScrollSnapList
    final double intendedPixel = index * widget.itemSize;
    final int difference = (intendedPixel - currentPixel).toInt();

    return (difference == 0) ? 1.0 : widget.dynamicItemOpacity ?? 1.0;
  }

Add .toInt() to final int difference = (intendedPixel - currentPixel).toInt();

Use int is better here.