Reduce duplicates of channel struct

Add/Fix custom clock limit defines (for YMF278B)

src/engine/platform/pet.h

@@ -24,10 +24,9 @@
 #include "../macroInt.h"
 
 class DivPlatformPET: public DivDispatch {
-  struct Channel {
-    int freq, baseFreq, pitch, pitch2, note, ins;
-    bool active, insChanged, freqChanged, keyOn, keyOff, inPorta, enable;
-    int vol, outVol, wave;
+  struct Channel: public SharedChannelFreq, public SharedChannelVolume<int> {
+    bool enable;
+    int wave;
     unsigned char sreg;
     int cnt;
     short out;
@@ -37,21 +36,9 @@ class DivPlatformPET: public DivDispatch {
       pitch2=0;
     }
     Channel():
-      freq(0),
-      baseFreq(0),
-      pitch(0),
-      pitch2(0),
-      note(0),
-      ins(-1),
-      active(false),
-      insChanged(true),
-      freqChanged(false),
-      keyOn(false),
-      keyOff(false),
-      inPorta(false),
+      SharedChannelFreq(),
+      SharedChannelVolume<int>(1),
       enable(false),
-      vol(1),
-      outVol(1),
       wave(0b00001111),
       sreg(0),
       cnt(0),