[opus] The inband FEC option within Opus

[Timothy B. Terriberry]

Jean-Marc Valin wrote:
> The -inbandfec option means that the encoder is *allowed* to use FEC,
> not that it has to. The final decision is based on the bitrate and the
> percentage of packet loss (and a few minor factors). If you're not
> seeing a difference, it means that given the options you're using, the
> encoder thinks actually using FEC would result in worse quality.

The most easily-overlooked option probably being -loss