Reduce duplicates of channel struct

Add/Fix custom clock limit defines (for YMF278B)

src/engine/platform/zxbeeper.h

@@ -25,11 +25,7 @@
 #include "../macroInt.h"
 
 class DivPlatformZXBeeper: public DivDispatch {
-  struct Channel {
-    int freq, baseFreq, pitch, pitch2, note;
-    int ins;
-    bool active, insChanged, freqChanged, keyOn, keyOff, inPorta;
-    signed char vol, outVol;
+  struct Channel: public SharedChannelFreq, public SharedChannelVolume<signed char> {
     unsigned short sPosition;
     unsigned char duty;
     DivMacroInt std;
@@ -38,20 +34,8 @@ class DivPlatformZXBeeper: 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),
-      vol(1),
-      outVol(1),
+      SharedChannelFreq(),
+      SharedChannelVolume<signed char>(1),
       sPosition(0),
       duty(64) {}
   };