Implement IFrameEncoder / IFrameDecoder using libopus

[gavv]

We have two interfaces IFrameEncoder and IFrameDecoder that are used to encode audio samples into packet payload and decode it back. Currently they're implemented using PcmEncoder and PcmDecoder (for uncompressed PCM).

Now we need to add two more implementations: OpusEncoder and OpusDecoder, that will use libopus library to do the job.

We also need to add new encoder & decoder to the list of tested codecs in test_frame_encoder_decoder.cpp.

[Nystana]

Hello I'd like to try this issue

[gavv]

Great, thank you!

[gavv]

@Nystana Please check out discussion here: #758

It uncovered a few gotchas that are likely the same for opus and vorbis (related to variable packet size), which were not addressed yet.

@runei Leonardo, what are your thoughts/plans regarding those changes needed in encoder/decoder interfaces? Should we wait until that is resolved in libvorbis PR, or work on it in libopus PR?

[runei]

Hi @gavv

I didn't work on this through the past weeks as I have been busy, so I can't give many insights abouts your comments yet. If you want to work in this on libopus, thats fine, I can wait and continue later. Or if you want to wait, I will continue this when I have more time.
You and @Nystana can decide what you prefer, I'm fine with both options.

[baranovmv]

I'm working on enabling FEC blocks to contain packets of arbitrary length. This is a minor change that won't break backward compatibility, but iframe[encoder|decoder] will require it.