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/Actor.cpp

@@ -671,12 +671,9 @@ void Actor::Command( CString sCommandString )
 
 float Actor::GetCommandLength( CString command )
 {
-	class NullActor: public Actor
-	{
-		void DrawPrimitives() { }
-	} temp;
-
+	Actor temp;
 	temp.Command(command);
+
 	return temp.GetTweenTimeLeft();
 }