Add chaining method for setting scene index

Author: adamgraham

Runtime/PropertyChaining.cs

@@ -58,6 +58,19 @@ public static T SetId<T>(this T tween, int id) where T: Tween
             return tween;
         }
 
+        /// <summary>
+        /// Sets the scene index of the tween to the given value.
+        /// </summary>
+        /// <param name="tween">The tween to assign the value to.</param>
+        /// <param name="index">The scene index to set.</param>
+        /// <returns>The tween itself to allow for chaining.</returns>
+        /// <typeparam name="T">The type of the tween.</typeparam>
+        public static T SetSceneIndex<T>(this T tween, int index) where T: Tween
+        {
+            tween.sceneIndex = index;
+            return tween;
+        }
+
         /// <summary>
         /// Sets the ease of the tween to the given value.
         /// </summary>