Add support for only rendering effects inside screen bounds

OpenRA.Game/Effects/IEffect.cs

@@ -20,5 +20,8 @@ namespace OpenRA.Effects
 		IEnumerable<IRenderable> Render(WorldRenderer r);
 	}
 
+	// Identifier interface for effects that are added to ScreenMap
+	public interface ISpatiallyPartitionable { }
+
 	public interface IEffectAboveShroud { IEnumerable<IRenderable> RenderAboveShroud(WorldRenderer wr); }
 }