Reduce duplicates of channel struct

Add/Fix custom clock limit defines (for YMF278B)

src/engine/platform/pcspkr.h

@@ -28,36 +28,15 @@
 #include <condition_variable>
 
 class DivPlatformPCSpeaker: public DivDispatch {
-  struct Channel {
-    int freq, baseFreq, pitch, pitch2, note, ins;
-    unsigned char duty, sweep;
-    bool active, insChanged, freqChanged, sweepChanged, keyOn, keyOff, inPorta, furnaceDac;
-    signed char vol, outVol, wave;
+  struct Channel: public SharedChannelFreq, public SharedChannelVolume<signed char> {
     DivMacroInt std;
     void macroInit(DivInstrument* which) {
       std.init(which);
       pitch2=0;
     }
     Channel():
-      freq(0),
-      baseFreq(0),
-      pitch(0),
-      pitch2(0),
-      note(0),
-      ins(-1),
-      duty(0),
-      sweep(8),
-      active(false),
-      insChanged(true),
-      freqChanged(false),
-      sweepChanged(false),
-      keyOn(false),
-      keyOff(false),
-      inPorta(false),
-      furnaceDac(false),
-      vol(15),
-      outVol(15),
-      wave(-1) {}
+      SharedChannelFreq(),
+      SharedChannelVolume<signed char>(15) {}
   };
   Channel chan[1];
   DivDispatchOscBuffer* oscBuf;