Fill image pixel low bits with zeros to test image quality loss and LSB steganography effects.
image[pixelIdx][0] = image[pixelIdx][0] >> bits << bits)
image[pixelIdx][1] = image[pixelIdx][1] >> bits << bits)
image[pixelIdx][2] = image[pixelIdx][2] >> bits << bits)
image[pixelIdx][3] = image[pixelIdx][3] >> bits << bits)