Ensure the background is eventually displayed when `IntroTriangles` suspends

osu.Game/Screens/Menu/IntroTriangles.cs

@@ -93,6 +93,9 @@ public override void OnSuspending(IScreen next)
         {
             base.OnSuspending(next);
 
+            // ensure the background is shown, even if the TriangleIntroSequence failed to do so.
+            background.ApplyToBackground(b => b.Show());
+
             // important as there is a clock attached to a track which will likely be disposed before returning to this screen.
             intro.Expire();
         }