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/engine.h

@@ -8,7 +8,7 @@
 struct DivChannelState {
   std::vector<DivDelayedCommand> delayed;
   int note, pitch, portaSpeed, portaNote;
-  int volume, volSpeed, cut, rowDelay;
+  int volume, volSpeed, cut, rowDelay, volMax;
   int vibratoDepth, vibratoRate, vibratoPos;
   int tremoloDepth, tremoloRate, tremoloPos;
   unsigned char arp, arpStage;