Expose raw Opus data

[abextm]

It is desirable in some situations to have access to audio data without it being decoded first. This is not possible with the current exposed API. This is incredibly useful for storing audio in memory as PCM data is significantly larger than the Opus packets.

[porjo]

I would find this useful too. I'm playing with WebRTC to create a listen-only client in a webpage, using gumble as a gateway.

I've hacked gumble to bypass the decode stream - see my branch