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/su.cpp

@@ -234,7 +234,7 @@ void DivPlatformSoundUnit::tick(bool sysTick) {
           if (sample->centerRate<1) {
             off=0.25;
           } else {
-            off=(double)sample->centerRate/(8363.0*4.0);
+            off=(double)sample->centerRate/(parent->getCenterRate()*4.0);
           }
           chan[i].freq=(double)chan[i].freq*off;
         }