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 `fcTl` and `fdAt` chunks have a 4 byte sequence number. Both chunk types share the sequence. The purpose of this number is to detect (and optionally correct) sequence errors in an Animated PNG, since the PNG specification does not impose orderng restrictions on ancillary chunks.