Reduce duplicates of channel struct

Add/Fix custom clock limit defines (for YMF278B)

src/engine/platform/vic20.h

@@ -26,32 +26,16 @@
 #include <queue>
 
 class DivPlatformVIC20: public DivDispatch {
-  struct Channel {
-    int freq, baseFreq, pitch, pitch2, note, ins;
-    unsigned char pan;
-    bool active, insChanged, freqChanged, keyOn, keyOff, inPorta;
-    int vol, outVol, wave, waveWriteCycle;
+  struct Channel: public SharedChannelFreq, public SharedChannelVolume<int> {
+    int wave, waveWriteCycle;
     DivMacroInt std;
     void macroInit(DivInstrument* which) {
       std.init(which);
       pitch2=0;
     }
     Channel():
-      freq(0),
-      baseFreq(0),
-      pitch(0),
-      pitch2(0),
-      note(0),
-      ins(-1),
-      pan(255),
-      active(false),
-      insChanged(true),
-      freqChanged(false),
-      keyOn(false),
-      keyOff(false),
-      inPorta(false),
-      vol(15),
-      outVol(15),
+      SharedChannelFreq(),
+      SharedChannelVolume<int>(15),
       wave(0),
       waveWriteCycle(-1) {}
   };