Changes

Note: For purposes of chunk descriptions, an "unsigned int" shall be a 32-bit unsigned integer in network byte order; a "signed int" shall be a 32-bit signed integer in network byte order; an "unsigned short" shall be a 16-bit unsigned integer in network byte order; a "byte" shall be a 8-bit unsigned integer.

The `aCTL` chunk is an ancillary chunk as defined in the PNG Specification. It must appear before the first `IDAT` chunk within a valid PNG stream.