Bug1714, more: better calculation of MidiTime...

... after I reflected more on the explanation of it in the long comment at top.

Brings back the use of PaUtil_GetTime() but now calls it also in the audio IO
callback, so we can correct the unspecified origin of times supplied to the
audio IO callback to agree with the PaUtilGetTime() clock.

Thus the Midi time calculation is again based on the clock time of the other
thread that calls MidiTime, making it a few milliseconds more accurate, while
avoiding subtraction of two times based on widely different origins, which
made the big numbers that overlowed and caused Bug1714 to happen.

src/AudioIO.h

@@ -530,7 +530,10 @@ private:
 
    // These fields are used to synchronize MIDI with audio:
 
-   /// PortAudio's currentTime
+   /// PortAudio's clock time
+   volatile double  mAudioCallbackClockTime;
+   /// PortAudio's currentTime -- its origin is unspecified!  So that's why
+   /// we also record the above
    volatile double  mAudioCallbackOutputCurrentTime;
    /// PortAudio's outTime
    volatile double  mAudioCallbackOutputDacTime;