Don't evaluate the ActorScroller transform function every time though Draw.

Cache TweenState values at a provided interval, then interpolate between the TweenStates.
TODO: Precalculate these TweenStates in Init?

stepmania/src/ScreenSelectMaster.h

@@ -55,6 +55,7 @@ protected:
 	ThemeMetric<float>		SCROLLER_SECONDS_PER_ITEM;
 	ThemeMetric<float>		SCROLLER_NUM_ITEMS_TO_DRAW;
 	ThemeMetric<CString>	SCROLLER_TRANSFORM;
+	ThemeMetric<int>		SCROLLER_SUBDIVISIONS;
 	ThemeMetric<CString>	DEFAULT_CHOICE;
 
 	int m_Next[NUM_MENU_DIRS][MAX_CHOICES];