better volume slide accuracy

volume slide seems to be kind of quirky:
- sliding all the way down and then up suddenly sets the vol to max
- apparently there is an overflow and the slide stops when it reaches
  its boundaries (instead of continuing)

src/engine/platform/genesisext.cpp

@@ -20,6 +20,9 @@ int DivPlatformGenesisExt::dispatch(DivCommand c) {
     case DIV_CMD_VOLUME:
       chan[c.chan].vol=c.value;
       break;
+    case DIV_CMD_GET_VOLMAX:
+      return 127;
+      break;
     default:
       break;
   }
@@ -35,4 +38,4 @@ int DivPlatformGenesisExt::init(DivEngine* parent, int channels, int sugRate) {
 
   extMode=true;
   return 13;
-}
+}