volume: Fix hesitating motion in popover on scroll wheel events

Scrolling on the volume widget changes the volume but it was hesitating
in certain cases after a few events one direction and then dialing back.
Fix this by queueing redraw frames while the animation is running.

src/panel/widgets/volume.cpp

@@ -14,6 +14,7 @@ WayfireVolumeScale::WayfireVolumeScale()
             value_changed.block();
             this->set_value(this->current_volume);
             value_changed.unblock();
+            this->queue_draw();
         }
     }, true);