prepare to add SMS platform

plays but no macro yet

src/engine/platform/sms.h

@@ -0,0 +1,21 @@
+#include "../dispatch.h"
+#include "sound/sn76496.h"
+
+class DivPlatformSMS: public DivDispatch {
+  struct Channel {
+    int freq, baseFreq, pitch;
+    unsigned char ins, note;
+    bool active, insChanged, freqChanged, keyOn, keyOff;
+    signed char vol;
+    Channel(): freq(0), baseFreq(0), pitch(0), ins(0), note(0), active(false), insChanged(true), freqChanged(false), keyOn(false), keyOff(false), vol(15) {}
+  };
+  Channel chan[4];
+  unsigned char snNoiseMode;
+  bool updateSNMode;
+  sn76496_device* sn;
+  public:
+    void acquire(short& l, short& r);
+    int dispatch(DivCommand c);
+    void tick();
+    int init(DivEngine* parent, int channels, int sugRate);
+};