Send an RTCP packet to report picture loss.

async def _send_rtcp_pli(self, media_ssrc):
        """
        Send an RTCP packet to report picture loss.
        """
        if self.__rtcp_ssrc is not None:
            packet = RtcpPsfbPacket(fmt=RTCP_PSFB_PLI, ssrc=self.__rtcp_ssrc, media_ssrc=media_ssrc)
            await self._send_rtcp(packet)