slider: Ignore left/right scroll directions

Both of these ended up with a "undefined" delta, which turned
the final value into 0 no matter the previous state.

While this was already fixed for touchpads in the previous commit
(by not forwarding emulated discrete events), this looks somewhat
out of place even for the mice that have left/right discrete scroll
actions (e.g. tilting the scroll wheel left or right).

Let these unhandled directions have a delta of 0, and focus on
vertical scroll for both smooth and discrete events.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/2967>

js/ui/slider.js

@@ -141,7 +141,7 @@ export const Slider = GObject.registerClass({
 
     scroll(event) {
         let direction = event.get_scroll_direction();
-        let delta;
+        let delta = 0;
 
         if (event.get_flags() & Clutter.EventFlags.FLAG_POINTER_EMULATED)
             return Clutter.EVENT_PROPAGATE;