This was due to context-level mainloop being accessed from different
audio threads at the same time. I've fixed this by giving each device
their own pa_mainloop and pa_context objects.

I considered the idea of having only a single context-level mainloop
and just using a mutex for mutal exclusion, but that would involve a
lock in the audio thread's data loop which I wasn't happy about. Also,
I wasn't sure which thread PulseAudio callbacks would get fired from
since the main loop would be iterated on different audio threads.

Public issue https://github.com/mackron/miniaudio/issues/376

Public issue https://github.com/mackron/miniaudio/issues/334

This replaces the stop callback. The new callback supports different
event types, not all of which are supported on all backends.

This commit also fixes a bug where the stop callback is not fired.

Public issue https://github.com/mackron/miniaudio/issues/351

Public issue https://github.com/mackron/miniaudio/issues/384

  * Add support for setting the directional attenuation factor. This is
    useful for allowing other effects to control the directional
    attenuation, such as HRTF effects.

  * Add a function for retrieving the index of the listener that a
    sound will be spatialized against based on the current state. A
    function to retrieve the direction to that listener has also been
    added. This is useful for when an effect needs to know information
    about the listener for some kind of spatialization.

  * A bug has been fixed where silence is output if a node has zero
    inputs.

  * A bug has been fixed where a bad channel map can be used for
    spatialization.

This option is intentionally undocumented and should be used at your
own risk.

Public issue: https://github.com/mackron/miniaudio/issues/294

Public enum https://github.com/mackron/miniaudio/issues/218

Public issue https://github.com/mackron/miniaudio/issues/218

fix a typo when using ma_round_to_standard_sample_rate__opensl.

Reorder examples in README

Public issue https://github.com/mackron/miniaudio/issues/218

fix wrong assert in ma_open_stream__aaudio

Public issue https://github.com/mackron/miniaudio/issues/379

Public issue https://github.com/mackron/miniaudio/issues/379

This happens when the pEngine parameter is NULL.