volume: fix mousewheel handling

Mouse scroll events are now consumed by vol faders,
and hence not passed on to the next widget.

src/avtk/volume.cxx

@@ -214,6 +214,9 @@ int Volume::handle(int event)
       return 1;
     case FL_ENTER:
       return 1;
+    case FL_MOUSEWHEEL:
+      //printf("no scroll on faders\n");
+      return 1;
     case FL_RELEASE:
       if (highlight) {
         highlight = 0;