Reduce duplicates of channel struct

Add/Fix custom clock limit defines (for YMF278B)

src/engine/platform/vb.h

@@ -27,12 +27,10 @@
 #include "sound/vsu.h"
 
 class DivPlatformVB: public DivDispatch {
-  struct Channel {
-    int freq, baseFreq, pitch, pitch2, note;
-    int ins;
+  struct Channel: public SharedChannelFreq, public SharedChannelVolume<signed char> {
     unsigned char pan, envLow, envHigh;
-    bool active, insChanged, freqChanged, keyOn, keyOff, inPorta, noise, deferredWaveUpdate;
-    signed char vol, outVol, wave;
+    bool noise, deferredWaveUpdate;
+    signed short wave;
     DivMacroInt std;
     DivWaveSynth ws;
     void macroInit(DivInstrument* which) {
@@ -40,25 +38,13 @@ class DivPlatformVB: public DivDispatch {
       pitch2=0;
     }
     Channel():
-      freq(0),
-      baseFreq(0),
-      pitch(0),
-      pitch2(0),
-      note(0),
-      ins(-1),
+      SharedChannelFreq(),
+      SharedChannelVolume<signed char>(15),
       pan(255),
       envLow(0),
       envHigh(0),
-      active(false),
-      insChanged(true),
-      freqChanged(false),
-      keyOn(false),
-      keyOff(false),
-      inPorta(false),
       noise(false),
       deferredWaveUpdate(false),
-      vol(15),
-      outVol(15),
       wave(-1) {}
   };
   Channel chan[6];