The SharedSingleProducerCircularQueue used here has dubious value, This
queue is used to pass commands to the audio thread, such as play/pause/
seek/volume change/etc. We can make do with a simple locked vector, as
we were blocking to enqueue tasks anyways. We can also use an atomic
bool to tell the audio thread when it needs to take a lock on the task
queue, to keep the thread lock-free most of the time.
Instead of having to duplicate the audio stream backend conditions, just
define PlaybackStream::create in each audio backend implementation file.
We provide a weak definition in PlaybackStream.cpp as the fallback.
