Fixed IDisposable implementation and usage.

- Implement IDisposable interface correctly, with sealed classes where possible for simplicity.
- Add using statement around undisposed local variables.

OpenRA.Mods.RA/Scripting/LuaScriptInterface.cs

@@ -32,7 +32,7 @@ namespace OpenRA.Mods.RA.Scripting
 		public object Create(ActorInitializer init) { return new LuaScriptInterface(this); }
 	}
 
-	public class LuaScriptInterface : IWorldLoaded, ITick
+	public sealed class LuaScriptInterface : IWorldLoaded, ITick, IDisposable
 	{
 		World world;
 		SpawnMapActors sma;
@@ -90,6 +90,11 @@ namespace OpenRA.Mods.RA.Scripting
 				context.InvokeLuaFunction("Tick");
 		}
 
+		public void Dispose()
+		{
+			context.Dispose();
+		}
+
 		[LuaGlobal]
 		public object New(string typeName, LuaTable args)
 		{