workspacesView: Clip the SecondaryMonitorDisplay instead of the view

Make the SecondaryMonitorDisplay a bit more similar to the
ControlsManager container on the primary monitor, and clip that widget
instead of the WorkspacesViews on secondary monitors.

This will allow us to overpaint the WorkspacesView allocation and paint
the WindowPreview overlays like the title and close button outside the
allocation with the next commit.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/1813>

js/ui/workspacesView.js

@@ -28,7 +28,6 @@ var WorkspacesViewBase = GObject.registerClass({
     _init(monitorIndex, overviewAdjustment) {
         super._init({
             style_class: 'workspaces-view',
-            clip_to_allocation: monitorIndex !== Main.layoutManager.primaryIndex,
             x_expand: true,
             y_expand: true,
         });
@@ -615,6 +614,7 @@ class SecondaryMonitorDisplay extends St.Widget {
                 index: this._monitorIndex,
                 work_area: true,
             }),
+            clip_to_allocation: true,
         });
 
         this.connect('destroy', () => this._onDestroy());