This is based on a patch from Ian Scott in 2014. It was never committed,
so I figured I'd fix the outstanding issue and resubmit it.

https://www.mail-archive.com/mpd-devel%40musicpd.org/msg00139.html

Eliminate the duplicate pa_operation_get_state() call.

There is no advantage in using g_setenv().

Passing a void pointer is unsafe.

Make it "const".

Prepare to migrate away from GLib.  Currently, we're still using GLib
as a backend.

Replaces GLib's GError.

Remove all #ifdefs.  Old versions of libpulse are not being tested,
and thus I'm removing support.

Reduce command latency while paused.

One large loop and only one pa_stream_get_state() call.

Pass audio_output objects around instead of void pointers.  This will
give some more control to the plugin, and prepares for non-blocking
audio outputs.

Provide _lock() and _unlock() to wrap all accesses from the mixer
plugin.

Always lock the main loop when operating on PULSE objects.  Document
this.

Fixes assertion failure when a stream callback is invoked too late
after a format change.

Merge common code.

Eliminate duplicate code.

Check if the stream is suspended; wake up the main loop when it
becomes suspended.

Not a bool.

Unlock the mainloop in all code paths.