Sometimes PNG image has more than one IDAT chunk.

Currently zopfli_png_recompress makes recompressed PNG image have single IDAT chunk.

After apply this patch, 

 * If a PNG image has more than one IDAT chunk, zopfli_png_recompress makes a recompressed PNG image have multiple IDAT chunks.
 * zopfli_png_recompress sets the compressed(with zopfli) IDAT chunk size the length of the first IDAT chunk.

This patch is for the streaming.