Add missing equality implementation for seasonal backgrounds

The equality operator is used to determine whether the next background
in the cycle should be loaded, to avoid pointless loads of the same
background several times (see #13362 and #13393). Its omission in the
latter pull caused seasonal backgrounds to no longer cycle.

Closes #13508.

osu.Game/Graphics/Backgrounds/SeasonalBackgroundLoader.cs

@@ -99,5 +99,14 @@ namespace osu.Game.Graphics.Backgrounds
             // ensure we're not loading in without a transition.
             this.FadeInFromZero(200, Easing.InOutSine);
         }
+
+        public override bool Equals(Background other)
+        {
+            if (ReferenceEquals(null, other)) return false;
+            if (ReferenceEquals(this, other)) return true;
+
+            return other.GetType() == GetType()
+                   && ((SeasonalBackground)other).url == url;
+        }
     }
 }