workspacesView: Update visibily when gesture drag begins

When dragging the workspaces through the swipe gesture, all
workspaces must be visible. WorkspacesView's _updateVisibility()
method special-cases this and ensures that.

However, this method is only called when (1) going to the active
workspace, and (2) when the gesture ends. That means, if there
is any workspace hidden by the time a gesture starts, it is never
shown!

Call _updateVisibility() on startTouchGesture() as well.

Related: https://gitlab.gnome.org/GNOME/gnome-shell/-/issues/2969

https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/1360

js/ui/workspacesView.js

@@ -241,6 +241,8 @@ class WorkspacesView extends WorkspacesViewBase {
 
     startTouchGesture() {
         this._gestureActive = true;
+
+        this._updateVisibility();
     }
 
     endTouchGesture() {