audio/out: feed AOs from a separate thread

This has 2 goals:
- Ensure that AOs have always enough data, even if the device buffers
  are very small.
- Reduce complexity in some AOs, which do their own buffering.

One disadvantage is that performance is slightly reduced due to more
copying.

Implementation-wise, we don't change ao.c much, and instead "redirect"
the driver's callback to an API wrapper in push.c.

Additionally, we add code for dealing with AOs that have a pull API.
These AOs usually do their own buffering (jack, coreaudio, portaudio),
and adding a thread is basically a waste. The code in pull.c manages
a ringbuffer, and allows callback-based AOs to read data directly.

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