Function stbi__compute_y_16

lite/example/cpp_example/mge/cv/stb_image.h:1883–1885

// nothing
#else
static stbi__uint16 stbi__compute_y_16(int r, int g, int b) {
    return (stbi__uint16)(((r * 77) + (g * 150) + (29 * b)) >> 8);
}
#endif

#if defined(STBI_NO_PNG) && defined(STBI_NO_PSD)