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.cpp

@@ -697,6 +697,11 @@ bool DivEngine::init() {
   blip_set_rates(bb[0],dispatch->rate,got.rate);
   blip_set_rates(bb[1],dispatch->rate,got.rate);
 
+  for (int i=0; i<chans; i++) {
+    chan[i].volMax=dispatch->dispatch(DivCommand(DIV_CMD_GET_VOLMAX,i))<<8;
+    chan[i].volume=chan[i].volMax;
+  }
+
   if (!output->setRun(true)) {
     logE("error while activating!\n");
     return false;