Can specify a minimum to maintain in the playback RingBuffer...

... this may be more than the batch size used in ongoing playback.

It is expected that this larger batch size is used only once when priming
the queue before starting play.  But then FillBuffers() may attempt to
refill up to the minimum in case demand is outpacing supply.

Thus the new number defines a "yellow zone" for the queue.

src/AudioIO.h

@@ -720,7 +720,12 @@ private:
    double              mSeek;
    double              mPlaybackRingBufferSecs;
    double              mCaptureRingBufferSecs;
+
+   /// Preferred batch size for replenishing the playback RingBuffer
    size_t              mPlaybackSamplesToCopy;
+   /// Occupancy of the queue we try to maintain, with bigger batches if needed
+   size_t              mPlaybackQueueMinimum;
+
    double              mMinCaptureSecsToCopy;
    /// True if audio playback is paused
    bool                mPaused;