Game Boy: I hate your artificial limitations

fixes a DefleMask demo module

src/engine/platform/gb.cpp

@@ -573,6 +573,10 @@ void DivPlatformGB::reset() {
   antiClickWavePos=0;
 }
 
+int DivPlatformGB::getPortaFloor(int ch) {
+  return 24;
+}
+
 bool DivPlatformGB::isStereo() {
   return true;
 }

src/engine/platform/gb.h

@@ -94,6 +94,7 @@ class DivPlatformGB: public DivDispatch {
     void forceIns();
     void tick(bool sysTick=true);
     void muteChannel(int ch, bool mute);
+    int getPortaFloor(int ch);
     bool isStereo();
     void notifyInsChange(int ins);
     void notifyWaveChange(int wave);