Recover initial packet from a retransmission packet.

def unwrap_rtx(rtx, payload_type, ssrc):
    """
    Recover initial packet from a retransmission packet.
    """
    packet = RtpPacket(
        payload_type=payload_type,
        marker=rtx.marker,
        sequence_number=unpack('!H', rtx.payload[0:2])[0],
        timestamp=rtx.timestamp,
        ssrc=ssrc,
        payload=rtx.payload[2:])
    packet.csrc = rtx.csrc
    packet.extensions = rtx.extensions
    return packet