generic screen popping: pop if pushed, instead of NEXT_SCREEN

stepmania/src/Screen.cpp

@@ -150,11 +150,17 @@ void Screen::HandleScreenMessage( const ScreenMessage SM )
 			MenuStart( p );
 		break;
 	case SM_GoToNextScreen:
-		SCREENMAN->SetNewScreen( NEXT_SCREEN );
+		if( SCREENMAN->IsStackedScreen(this) )
+			SCREENMAN->PopTopScreen();
+		else
+			SCREENMAN->SetNewScreen( NEXT_SCREEN );
 		break;
 	case SM_GoToPrevScreen:
 		SCREENMAN->DeletePreparedScreens();
-		SCREENMAN->SetNewScreen( PREV_SCREEN );
+		if( SCREENMAN->IsStackedScreen(this) )
+			SCREENMAN->PopTopScreen();
+		else
+			SCREENMAN->SetNewScreen( PREV_SCREEN );
 		break;
 	}
 }

stepmania/src/ScreenManager.cpp

@@ -137,6 +137,15 @@ Screen *ScreenManager::GetTopScreen()
 	return m_ScreenStack[m_ScreenStack.size()-1];
 }
 
+bool ScreenManager::IsStackedScreen( const Screen *pScreen ) const
+{
+	/* True if the screen is in the screen stack, but not the first. */
+	for( unsigned i = 1; i < m_ScreenStack.size(); ++i )
+		if( m_ScreenStack[i] == pScreen )
+			return true;
+	return false;
+}
+
 void ScreenManager::Update( float fDeltaTime )
 {
 	// Only update the topmost screen on the stack.

stepmania/src/ScreenManager.h

@@ -63,6 +63,11 @@ public:
 
 	Screen *GetTopScreen();
 
+	/* Return true if the given screen is in the main screen stack, but not the bottommost
+	 * screen.  If true, the screen should usually exit by popping itself, not by loading
+	 * another screen. */
+	bool IsStackedScreen( const Screen *pScreen ) const;
+
 	// Lua
 	void PushSelf( lua_State *L );