Merge "Encode paletted PNGs more efficiently (part 2)"

        png_bytep row = imageInfo.rows[j];

        png_bytep out = outRows[j];

        for (i = 0; i < w; i++) {

            rr = *row++;

            gg = *row++;

            bb = *row++;

            aa = *row++;

            // Make sure any zero alpha pixels are fully zeroed.  On average,

            // each of our PNG assets seem to have about four distinct pixels

            // with zero alpha.

            // There are several advantages to setting these to zero:

            // (1) Images are more likely able to be encodable with a palette.

            // (2) Image palettes will be smaller.

            // (3) Premultiplied and unpremultiplied PNG decodes can skip

            //     writing zeros to memory, often saving significant numbers

            //     of memory pages.

            aa = *(row + 3);

            if (aa == 0) {

                rr = 0;

                gg = 0;

                bb = 0;

                // Also set red, green, and blue to zero in "row".  If we later

                // decide to encode the PNG as RGB or RGBA, we will use the

                // values stored there.

                *(row) = 0;

                *(row + 1) = 0;

                *(row + 2) = 0;

            } else {

                rr = *(row);

                gg = *(row + 1);

                bb = *(row + 2);

            }

            row += 4;

            int odev = maxGrayDeviation;

            maxGrayDeviation = MAX(ABS(rr - gg), maxGrayDeviation);