Ensures adain_npy returned pixel value is within the range of bit depth

[kennethxu]

This fixes #222. The normalization process of adain_npy could generate pixel values that are out of the range of original image bit depth. For example, it computed values less than 0 or greater than 256 for 8-bit images. When this happens, the later process will consider the image to be 16 bit depth. But since most of pixels are less than 256, which is effectively black in 16 bit (65536) space, the result is an entirely black image. The solution is to compute a proper ratio to reduce style_std so that no pixel would exceed the range of original bit depth while still apply the style in the best effort.