prepare to add function for mapping vol to percent

src/engine/dispatch.h

@@ -681,6 +681,14 @@ class DivDispatch {
      */
     virtual int mapVelocity(int ch, float vel);
 
+    /**
+     * map chip volume to gain.
+     * @param ch the chip channel. -1 means N/A.
+     * @param vol input volume.
+     * @return output gain fron 0.0 to 1.0.
+     */
+    virtual float getGain(int ch, int vol);
+
     /**
      * get the lowest note in a portamento.
      * @param ch the channel in question.

src/engine/platform/abstract.cpp

@@ -107,6 +107,11 @@ int DivDispatch::mapVelocity(int ch, float vel) {
   return round(vel*volMax);
 }
 
+float DivDispatch::getGain(int ch, int vol) {
+  const float volMax=MAX(1,dispatch(DivCommand(DIV_CMD_GET_VOLMAX,MAX(ch,0))));
+  return (float)vol/volMax;
+}
+
 int DivDispatch::getPortaFloor(int ch) {
   return 0x00;
 }