Implement null-op output path to use when can't load anything or audio disabled

[bvibber]

Needed for handling audioless autoplay in ogv.js -- bvibber/ogv.js#332

Should use this as graceful failover when AudioContext is disabled or when we can't load the Flash shim.

Needs to accept input and maintain a playback clock, basically drain data out of buffers in a timer?