Reduce duplicates of channel struct

Add/Fix custom clock limit defines (for YMF278B)

src/engine/platform/fmsharedbase.h

@@ -21,6 +21,8 @@
 #define _FMSHARED_BASE_H
 
 #include "../dispatch.h"
+#include "../instrument.h"
+#include "../macroInt.h"
 #include <deque>
 
 #define KVS(x,y) ((chan[x].state.op[y].kvs==2 && isOutput[chan[x].state.alg][y]) || chan[x].state.op[y].kvs==1)
@@ -46,6 +48,38 @@ class DivPlatformFMBase: public DivDispatch {
       0,2,1,3
     };
 
+    struct FMChannel: public SharedChannelFreq, public SharedChannelVolume<int> {
+      DivInstrumentFM state;
+      DivMacroInt std;
+      unsigned char freqH, freqL;
+      int portaPauseFreq;
+      unsigned char opMask;
+      signed char konCycles;
+      bool hardReset, opMaskChanged;
+
+      void macroInit(DivInstrument* which) {
+        std.init(which);
+        pitch2=0;
+      }
+      FMChannel():
+        SharedChannelFreq(),
+        SharedChannelVolume<int>(0),
+        freqH(0),
+        freqL(0),
+        portaPauseFreq(0),
+        opMask(15),
+        konCycles(0),
+        hardReset(false),
+        opMaskChanged(false) {}
+    };
+
+    struct FMChannelStereo: public FMChannel {
+      unsigned char pan;
+      FMChannelStereo():
+        FMChannel(),
+        pan(3) {}
+    };
+
     struct QueuedWrite {
       unsigned short addr;
       unsigned char val;