Audioinput streams are broken

[Manishearth]

examples/audioinput_stream.rs from #362 should produce loopback sound, it does not, at least on my and @ferjm's machine.

Some potentially useful debug info is at #362 (comment)

[Manishearth]

So it:

• works on @jdm's Mac
• does not work on @ferjm's Mac
• does not work on my 20.04 Ubuntu system that has latest gstreamer
• does work on my 18.04 Ubuntu system which is using servo's downloaded gstreamer

Seems like it works in servo's gstreamer, at least!