Add parameter --blend-on-white

[doterax]

inline uint8_t BlendOnWhite(const uint8_t val, const uint8_t alpha) {
if (alpha < 1) return 255 - ((static_cast(val) * static_cast(alpha) + 128) / 255);
return val;
}

Code and idea from https://github.com/google/guetzli/issues/184