Remove Invalidate and Restore from Actor, Screen and ScreenManager.

They don't do anything, and they're never called; I think they're
D3D hangovers.

Don't make Actor::DrawPrimitives pure virtual.  It's useful to make
Actors that don't draw anything, and only receive updates, to modularize
functionality, such as TimingTip.  This also makes Actor::GetCommandLength
somewhat less of a heinous hack.

stepmania/src/ScreenManager.cpp

@@ -25,6 +25,8 @@
 #include "Screen.h"
 #include "SongManager.h"
 
+#include "BitmapText.h"
+#include "Quad.h"
 
 ScreenManager*	SCREENMAN = NULL;	// global and accessable from anywhere in our program
 
@@ -333,21 +335,6 @@ void ScreenManager::Update( float fDeltaTime )
 }
 
 
-void ScreenManager::Restore()
-{
-	// Restore all CurrentScreens (back to front)
-	for( unsigned i=0; i<m_ScreenStack.size(); i++ )
-		m_ScreenStack[i]->Restore();
-	m_SystemLayer->Restore();
-}
-
-void ScreenManager::Invalidate()
-{
-	for( unsigned i=0; i<m_ScreenStack.size(); i++ )
-		m_ScreenStack[i]->Invalidate();
-	m_SystemLayer->Invalidate();
-}
-
 void ScreenManager::Draw()
 {
 	if(g_TopNeedsNeedsNullUpdate)