title: How to use FEC feature for Opus codec
body: <p>I'm trying to use the opus Forward Error Correction (FEC) feature.
  I have a service which does the encoding with OPUS_SET_INBAND_FEC(1)
  and OPUS_SET_PACKET_LOSS_PERC(20) with 10ms packets and sends them over UDP.</p>
  
  <p>I'm not clear on the decoding process though. </p>
  
  <ol>
  <li>When a packet is lost, do I need to call decode with fec=1 ONLY or do I need to call decode with fec=0 after as well on the next packet?  </li>
  <li>How do I know up front the size of the pcm that I send to decode with fec enabled?</li>
  </ol>