wait for the portaudio finished callback to avoid sporadic deadlocks

[m0dB]

This fixes #14055 . I describe the issue in detail in the bug report.

The fix consists of using Pa_SetStreamFinishedCallback to set a callback to wait for the stream to finish before continuing the main thread.

I have tested this by starting and stopping the devices programmatically in a loop. After 5000 iterations I still haven't had a deadlock, so I am confident the issue is fixed (without the fix I would get a deadlock every ~50 iterations).

Btw, there was this warning in the code (I have removed it):

        // BIG FAT WARNING: Pa_AbortStream() will kill threads while they're
        // waiting on a mutex, which will leave the mutex in an screwy
        // state. Don't use it!

This might be the same problem as described in #14055, and it is possible that with this fix we could also use Pa_AbortStream, instead of using the paAbort return value from the callback. But I didn't want to change the code more than necessary, and only add the wait condition, which is simple and safe.