workspace: Reset window opacity after overview transitions

When using the fade animation when transitioning to the overview instead
of zoom, we fade out all window previews to fully transparent. But after
commit 751189253a removed the old _updateWindowPositions() function,
nothing resets the opacity again, so when switching from the app- to the
window picker, all previews are hidden.

Fix this by always resetting the window preview opacity after showing
or hiding the overview.

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

js/ui/workspace.js

@@ -1194,10 +1194,12 @@ class Workspace extends St.Widget {
 
     _doneLeavingOverview() {
         this.layout_manager.layout_frozen = false;
+        this._windows.forEach(w => (w.opacity = 255));
     }
 
     _doneShowingOverview() {
         this.layout_manager.layout_frozen = false;
+        this._windows.forEach(w => (w.opacity = 255));
     }
 
     _isMyWindow(window) {