Set IsDisposed to true at the end of Scene.Dipose() (#202)

I noticed the Scene.Dispose() does not set IsDisposed = true at the end of the function.

Author: jordanlhunt

articles/tutorials/building_2d_games/17_scenes/snippets/scene.cs

@@ -112,6 +112,7 @@ protected virtual void Dispose(bool disposing)
             UnloadContent();
             Content.Dispose();
         }
+        IsDisposed = true; 
     }
     #endregion
 }