Give actors the ability to undo the rate change triggered by holding

tilde or tab.

This is gated by a new boolean, `tab_tilde_scaling_enabled_`, and
applied only to banners by default.

src/GameLoop.h

@@ -5,7 +5,8 @@ namespace GameLoop
 {
 	void RunGameLoop();
 	void UpdateAllButDraw( bool bRunningFromVBLANK);
-	void SetUpdateRate( float fUpdateRate );
+	void SetUpdateRate(float fUpdateRate);
+	float GetUpdateRate();
 	void ChangeTheme(const RString &sNewTheme);
 	void ChangeGame(const RString& new_game, const RString& new_theme= "");
 	void StartConcurrentRendering();