Added the OPUS_SET_INBAND_FEC(2) option

[yanhua0229]

What are the benefits of adding the OPUS_SET_INBAND_FEC(2) option? How does it affect audio quality? How does it affect audio quality compared to OPUS_SET_INBAND_FEC(1)?

[jmvalin]

With OPUS_SET_INBAND_FEC(1), the encoder will switch to SILK if there's packet loss because CELT does not support FEC. That's not great for music. With OPUS_SET_INBAND_FEC(2) the switch to SILK will be inhibited when the signal is content is music.