code: function crc32(png, offs, size) {
  					var crc = -1;
  					for (var i = 4; i < size-4; i += 1) {
  						crc = _crc32[(crc ^ png[offs+i].charCodeAt(0)) & 0xff] ^ ((crc >> 8) & 0x00ffffff);
  					}
  					write(png, offs+size-4, byte4(crc ^ -1));
  				}
docs: compute crc32 of the PNG chunks