prepare for proper center rate

the correct rate is 8372Hz rather than 8363Hz
sadly this will require a compat flag so I am not making this change
effective until 0.7

issue #2347

src/engine/platform/swan.cpp

@@ -185,7 +185,7 @@ void DivPlatformSwan::tick(bool sysTick) {
           if (s->centerRate<1) {
             off=1.0;
           } else {
-            off=8363.0/(double)s->centerRate;
+            off=parent->getCenterRate()/(double)s->centerRate;
           }
         }
         dacRate=((double)chipClock/2)/MAX(1,off*chan[i].freq);