Generate a (chunk_type, chunk_offset) 2-tuple for each of the chunks
        in the PNG image stream. Iteration stops after the IEND chunk is
        returned.

def _iter_chunk_offsets(self):
        """
        Generate a (chunk_type, chunk_offset) 2-tuple for each of the chunks
        in the PNG image stream. Iteration stops after the IEND chunk is
        returned.
        """
        chunk_offset = 8
        while True:
            chunk_data_len = self._stream_rdr.read_long(chunk_offset)
            chunk_type = self._stream_rdr.read_str(4, chunk_offset, 4)
            data_offset = chunk_offset + 8
            yield chunk_type, data_offset
            if chunk_type == 'IEND':
                break
            # incr offset for chunk len long, chunk type, chunk data, and CRC
            chunk_offset += (4 + 4 + chunk_data_len + 4)