Make polling loop period slightly shorter than Audio thread loop...

... but we don't need to make it as precise, using steady_clock.

Do this so that the Audio thread doesn't read the same polling message twice,
halting the scrub.  It's all right that it might miss one of the messages
instead.

src/AudioIO.cpp

@@ -1987,7 +1987,7 @@ int AudioIO::StartStream(const TransportTracks &tracks,
       // execute too much else
       if (mScrubState) {
          mOwningProject->GetScrubber().ContinueScrubbingPoll();
-         wxMilliSleep( Scrubber::ScrubPollInterval_ms );
+         wxMilliSleep( Scrubber::ScrubPollInterval_ms * 0.9 );
       }
       else
 #endif