Echo cancellation: Need to specify the source of the echo cancellation reference signal

[alvestrand]

At the moment, echo cancellation is defined on a MediaStreamTrack, but the source of the signal to be cancelled is not specified, leaving this up to the implementation.

Since most cases of echo are from a specific output device creating echo into an input device, it makes sense to specify for a given input (made visible as a MediaStreamTrack) to be echo-cancelled against the output that the application thinks will most affect it - most of the time, this will be the system default output device, but sometimes (as with headphones on the non-default device with mechanical-path echo), the right device is something else.

This seems to be addressable with a means of specifying which output device the input is to be echo-cancelled against; the most logical source of such identifiers is the output device ID from [[mediacapture-output]].

Tagging @o1ka