Allow audio-only streaming

[davedoesdev]

At the moment, video is assumed up to the encoder at least.

[davedoesdev]

Video can now be turned off (WebGL will be emitted by the canvas instead).

Streaming without a video track is a different matter.
webm-muxer.js does support it but this is more of a UI issue. We'd have to have a menu item to say don't stream video, and if checked then remove the camera icon and the canvas. Probably the same the other way round (disable audio).