FDS: set a post-amp value

src/engine/platform/fds.cpp

@@ -498,6 +498,10 @@ void DivPlatformFDS::notifyInsDeletion(void* ins) {
   }
 }
 
+float DivPlatformFDS::getPostAmp() {
+  return useNP?1.8f:1.0f;
+}
+
 void DivPlatformFDS::poke(unsigned int addr, unsigned short val) {
   rWrite(addr,val);
 }

src/engine/platform/fds.h

@@ -99,6 +99,7 @@ class DivPlatformFDS: public DivDispatch {
     void setNSFPlay(bool use);
     void setFlags(unsigned int flags);
     void notifyInsDeletion(void* ins);
+    float getPostAmp();
     void poke(unsigned int addr, unsigned short val);
     void poke(std::vector<DivRegWrite>& wlist);
     const char** getRegisterSheet();