Fix mask. Traditionally, quads have been a little unusual: they have a size

of 1x1, and are sized by zooming.  That's unintuitive, commonly causes
problems when used in conjunction with other stretch effects, and all we have
to do is SetHeight/SetWidth to set the actual size, which will make it behave
exactly like any other sprite.  This one didn't work because m_exprTransformFunction
overrides the zoom, usually resetting it to 1x1.

stepmania/src/ActorScroller.cpp

@@ -67,12 +67,8 @@ void ActorScroller::Load2(
 
 	ScrollThroughAllItems();
 
-	RectF rectBarSize(
-		-fItemWidth/2,
-		-fItemHeight/2,
-		fItemWidth/2,
-		fItemHeight/2 );
-	m_quadMask.StretchTo( rectBarSize );
+	m_quadMask.SetWidth( fItemWidth );
+	m_quadMask.SetHeight( fItemHeight );
 	m_quadMask.SetHidden( false );
 
 	m_bLoaded = true;