Splits a PNG file into chunks.
Result is an array of arrays,
({ ({ string chunk_type, string data, int crc_ok }),
({ string chunk_type, string data, int crc_ok }) ... })
chunk_type is the type of the chunk, like
"IHDR" or "IDAT".
data is the actual chunk data.
crcok is set to 1 if the checksum is ok and
dontcheckcrc parameter isn't set.
Returns 0 if it isn't a PNG file.