Return an offset, byte 2-tuple for the next byte in *stream* that is
        not '\xFF', starting with the byte at offset *start*. If the byte at
        offset *start* is not '\xFF', *start* and the returned *offset* will
        be the same.

def _next_non_ff_byte(self, start):
        """
        Return an offset, byte 2-tuple for the next byte in *stream* that is
        not '\xFF', starting with the byte at offset *start*. If the byte at
        offset *start* is not '\xFF', *start* and the returned *offset* will
        be the same.
        """
        self._stream.seek(start)
        byte_ = self._read_byte()
        while byte_ == b'\xFF':
            byte_ = self._read_byte()
        offset_of_non_ff_byte = self._stream.tell() - 1
        return offset_of_non_ff_byte, byte_