FM: better pitch slide accuracy

so... it appears that pitch slides in Genesis system aren't truly linear
instead, they are "kind of linear". the frequency isn't linear, and when
it overflows the range of an octave, the high bit changes and the
frequency is shifted right

src/engine/platform/genesis.cpp

@@ -133,6 +133,27 @@ void DivPlatformGenesis::tick() {
   psg.tick();
 }
 
+int DivPlatformGenesis::octave(int freq) {
+  if (freq>=82432) {
+    return 128;
+  } else if (freq>=41216) {
+    return 64;
+  } else if (freq>=20608) {
+    return 32;
+  } else if (freq>=10304) {
+    return 16;
+  } else if (freq>=5152) {
+    return 8;
+  } else if (freq>=2576) {
+    return 4;
+  } else if (freq>=1288) {
+    return 2;
+  } else {
+    return 1;
+  }
+  return 1;
+}
+
 #define rWrite(a,v) pendingWrites[a]=v;
 
 int DivPlatformGenesis::dispatch(DivCommand c) {
@@ -242,13 +263,13 @@ int DivPlatformGenesis::dispatch(DivCommand c) {
       int destFreq=644.0f*pow(2.0f,((float)c.value2/12.0f));
       bool return2=false;
       if (destFreq>chan[c.chan].baseFreq) {
-        chan[c.chan].baseFreq=(chan[c.chan].baseFreq*(960+c.value))/960;
+        chan[c.chan].baseFreq=chan[c.chan].baseFreq+c.value*octave(chan[c.chan].baseFreq);
         if (chan[c.chan].baseFreq>=destFreq) {
           chan[c.chan].baseFreq=destFreq;
           return2=true;
         }
       } else {
-        chan[c.chan].baseFreq=(chan[c.chan].baseFreq*(960-c.value))/960;
+        chan[c.chan].baseFreq=chan[c.chan].baseFreq-c.value*octave(chan[c.chan].baseFreq);
         if (chan[c.chan].baseFreq<=destFreq) {
           chan[c.chan].baseFreq=destFreq;
           return2=true;

src/engine/platform/genesis.h

@@ -43,6 +43,8 @@ class DivPlatformGenesis: public DivDispatch {
   
     short oldWrites[512];
     short pendingWrites[512];
+
+    int octave(int freq);
   
   public:
     void acquire(int& l, int& r);