use deque in Actor for contant time delete of the head

stepmania/src/Actor.cpp

@@ -261,8 +261,8 @@ void Actor::UpdateTweening( float fDeltaTime )
 			m_current = TS;
 			
 			// delete the head tween
-			m_TweenStates.erase( m_TweenStates.begin() );
-			m_TweenInfo.erase( m_TweenInfo.begin() );
+			m_TweenStates.pop_front();
+			m_TweenInfo.pop_front();
 		}
 		else		// in the middle of tweening.  Recalcute the current position.
 		{
@@ -380,8 +380,6 @@ void Actor::StopTweening()
 {
 	m_TweenStates.clear();
 	m_TweenInfo.clear();
-	ASSERT( m_TweenStates.empty() );
-	ASSERT( m_TweenInfo.empty() );
 }
 
 void Actor::FinishTweening()

stepmania/src/Actor.h

@@ -13,6 +13,7 @@
 
 #include "RageTypes.h"
 #include "ActorCommands.h"	// for ParsedCommand
+#include <deque>
 
 class Actor
 {
@@ -319,8 +320,8 @@ protected:
 	RageVector2	m_size;
 	TweenState	m_current;
 	TweenState	m_start;
-	vector<TweenState>	m_TweenStates;
-	vector<TweenInfo>	m_TweenInfo;
+	deque<TweenState>	m_TweenStates;	// use deque for contant time delete of the head
+	deque<TweenInfo>	m_TweenInfo;
 
 	TweenState& LatestTween() { ASSERT(m_TweenStates.size()>0);	return m_TweenStates.back(); };