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/dispatch.h

@@ -9,6 +9,7 @@ enum DivDispatchCmds {
   DIV_CMD_INSTRUMENT,
   DIV_CMD_VOLUME,
   DIV_CMD_GET_VOLUME,
+  DIV_CMD_GET_VOLMAX,
   DIV_CMD_NOTE_PORTA,
   DIV_CMD_PITCH,
   DIV_CMD_PANNING,