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/ga20.cpp

@@ -145,7 +145,7 @@ void DivPlatformGA20::tick(bool sysTick) {
         if (s->centerRate<1) {
           off=1.0;
         } else {
-          off=8363.0/s->centerRate;
+          off=parent->getCenterRate()/s->centerRate;
         }
       }
       DivSample* s=parent->getSample(chan[i].sample);