add EndScreen(), symmetric with BeginScreen

stepmania/src/Screen.h

@@ -45,6 +45,9 @@ public:
 	/* This is called immediately before the screen is used. */
 	virtual void BeginScreen();
 
+	/* This is called when the screen is popped. */
+	virtual void EndScreen() { }
+
 	virtual void Update( float fDeltaTime );
 	virtual bool OverlayInput( const InputEventPlus &input );
 	virtual void Input( const InputEventPlus &input );

stepmania/src/ScreenManager.cpp

@@ -323,6 +323,7 @@ ScreenMessage ScreenManager::PopTopScreenInternal( bool bSendLoseFocus )
 
 	if( bSendLoseFocus )
 		ls.m_pScreen->HandleScreenMessage( SM_LoseFocus );
+	ls.m_pScreen->EndScreen();
 
 	if( g_setPersistantScreens.find(ls.m_pScreen->GetName()) != g_setPersistantScreens.end() )
 	{