Add an option to delay screen loads until after we've deleted the

old screen.  This means that we may unload resources when we'll
need them immediately, but lowers the memory usage peak considerably
for very low-memory machines.

stepmania/src/ScreenManager.cpp

@@ -315,6 +315,7 @@ void ScreenManager::EmptyDeleteQueue()
  * change state!).  Only do this when we have to, so we don't double
  * the number of updates. */
 static bool g_TopNeedsNeedsNullUpdate = false;
+static bool g_SkipRendering = false;
 
 void ScreenManager::Update( float fDeltaTime )
 {
@@ -331,25 +332,54 @@ void ScreenManager::Update( float fDeltaTime )
 	 * If we cap that large update delta from the screen load, the update
 	 * to load the new screen will come after 4 seconds plus the load time.
 	 *
-	 * So, let's just drop the first update for every screen.
+	 * So, let's just zero the first update for every screen.
+	 *
+	 * XXX:  If a new Screen is set during this Update, that new screen is Drawn
+	 * before it's first Update.
 	 */
-	ASSERT( !m_ScreenStack.empty() );	// Why play the game if there is nothing showing?
-	Screen* pScreen = m_ScreenStack[m_ScreenStack.size()-1];
-	if( pScreen->IsFirstUpdate() )
-		pScreen->Update( 0 );
-	else
-		pScreen->Update( fDeltaTime );
+	ASSERT( !m_ScreenStack.empty() || m_DelayedScreen != "" );	// Why play the game if there is nothing showing?
+
+	if( !m_ScreenStack.empty() )
+	{
+		Screen* pScreen = m_ScreenStack[m_ScreenStack.size()-1];
+		if( pScreen->IsFirstUpdate() )
+			pScreen->Update( 0 );
+		else
+			pScreen->Update( fDeltaTime );
+	}
 
-	// TODO:  If a new Screen is set during this Update, that new screen is Drawn
-	// before it's first Update.
 	m_SystemLayer->Update( fDeltaTime );
 
 	EmptyDeleteQueue();
+
+	if(m_DelayedScreen.size() != 0)
+	{
+		LOG->Trace("loading");
+		/* We have a screen to display.  Delete the current screens and load it. */
+		m_ScreensToDelete.insert(m_ScreensToDelete.end(), m_ScreenStack.begin(), m_ScreenStack.end());
+		m_ScreenStack.clear();
+		EmptyDeleteQueue();
+		LoadDelayedScreen();
+
+		/* Ack.  We can't Update(0), since we want to skip the *next* update
+		 * (which is the one that will have all this load time in it).  We
+		 * can't draw it until we've updated it.  Let's simply not render
+		 * the next frame, so we'll come around quickly and handle it correctly. */
+		g_SkipRendering = true;
+	}
 }
 
 
 void ScreenManager::Draw()
 {
+	DISPLAY->Clear();
+
+	if(g_SkipRendering)
+	{
+		/* Leave the frame there (if any). */
+		g_SkipRendering = false;
+		return;
+	}
 	if(g_TopNeedsNeedsNullUpdate)
 	{
 		g_TopNeedsNeedsNullUpdate = false;
@@ -362,6 +392,8 @@ void ScreenManager::Draw()
 		for( unsigned i=0; i<m_ScreenStack.size(); i++ )	// Draw all screens bottom to top
 			m_ScreenStack[i]->Draw();
 	m_SystemLayer->Draw();
+
+	DISPLAY->Flip();
 }
 
 
@@ -422,18 +454,33 @@ void ScreenManager::SetNewScreen( Screen *pNewScreen )
 
 	// move current screen(s) to ScreenToDelete
 	m_ScreensToDelete.insert(m_ScreensToDelete.end(), m_ScreenStack.begin(), m_ScreenStack.end());
-
 	m_ScreenStack.clear();
+
 	m_ScreenStack.push_back( pNewScreen );
 }
 
 void ScreenManager::SetNewScreen( CString sClassName )
 {
+	m_DelayedScreen = sClassName;
+
+	/* If we're not delaying screen loads, load it now.  Otherwise, we'll load
+	 * it on the next iteration.  Only delay if we already have a screen
+	 * loaded; otherwise, there's no reason to delay. */
+	if(!PREFSMAN->m_bDelayedScreenLoad) // || m_ScreenStack.empty() )
+		LoadDelayedScreen();
+}
+
+void ScreenManager::LoadDelayedScreen()
+{
+retry:
+	CString sClassName = m_DelayedScreen;
+	m_DelayedScreen = "";
+
 	/* If we prepped a screen but didn't use it, nuke it. */
 	SAFE_DELETE( m_ScreenBuffered );
 
 	RageTimer t;
-	
+
 	Screen* pOldTopScreen = m_ScreenStack.empty() ? NULL : m_ScreenStack.back();
 
 	// It makes sense that ScreenManager should allocate memory for a new screen since it 
@@ -451,6 +498,13 @@ void ScreenManager::SetNewScreen( CString sClassName )
 		return;
 	}
 
+	if( PREFSMAN->m_bDelayedScreenLoad && m_DelayedScreen != "" )
+	{
+		/* Same deal: the ctor called SetNewScreen again.  Delete the screen
+		 * we just made, but don't delay again. */
+		SAFE_DELETE( pNewScreen );
+		goto retry;
+	}
 	SetNewScreen( pNewScreen );
 
 	/* If this is a system menu, don't let the operator key touch it!