Render to gray-scale about lunasvg

[sammycage]

Bitmap to GrayScale

    for (uint32_t y = 0; y < bitmap.height(); y++) {
        uint8_t* row = bitmap.data() + bitmap.stride() * y;
        for (uint32_t x = 0; x < bitmap.width(); x++) {
            uint32_t r = row[0];
            uint32_t g = row[1];
            uint32_t b = row[2];

            uint32_t l = (r * 299 + g * 587 + b * 114);

            row[0] = row[1] = row[2] = uint8_t(l / 1000);

            row += 4;
        }
    }