Reduce duplicates of channel struct

Add/Fix custom clock limit defines (for YMF278B)

src/engine/platform/ay8930.h

@@ -26,7 +26,7 @@
 
 class DivPlatformAY8930: public DivDispatch {
   protected:
-    struct Channel {
+    struct Channel: public SharedChannelFreq, public SharedChannelVolume<int> {
       struct Envelope {
         unsigned char mode;
         unsigned short period;
@@ -81,40 +81,23 @@ class DivPlatformAY8930: public DivDispatch {
           furnaceDAC(0) {}
       } dac;
 
-      int freq, baseFreq, note, pitch, pitch2;
-      int ins;
       unsigned char autoEnvNum, autoEnvDen, duty;
       signed char konCycles;
-      bool active, insChanged, freqChanged, keyOn, keyOff, portaPause, inPorta;
-      int vol, outVol;
       DivMacroInt std;
       void macroInit(DivInstrument* which) {
         std.init(which);
         pitch2=0;
       }
       Channel():
+        SharedChannelFreq(),
+        SharedChannelVolume<int>(31),
         envelope(Envelope()),
         curPSGMode(PSGMode(0)),
         nextPSGMode(PSGMode(1)),
         dac(DAC()),
-        freq(0),
-        baseFreq(0),
-        note(0),
-        pitch(0),
-        pitch2(0),
-        ins(-1),
         autoEnvNum(0),
         autoEnvDen(0),
-        duty(4),
-        active(false),
-        insChanged(true),
-        freqChanged(false),
-        keyOn(false),
-        keyOff(false),
-        portaPause(false),
-        inPorta(false),
-        vol(0),
-        outVol(31) {}
+        duty(4) {}
     };
     Channel chan[3];
     bool isMuted[3];