Gaaah. Head hurts.

Fix rounding error: when rect_width is 9, we want
rect_cx = pRect->left + 4.5, not 4.  This resulted in a gap between the
black quad and the gradient quad in TransitionFadeWipe, and possibly
other obscure things.  (The transition doesn't use a texture at all, so this
wasn't actually related to the bilinear filtering issue.)

stepmania/src/Actor.cpp

@@ -348,8 +348,8 @@ void Actor::StretchTo( LPRECT pRect )
 	int rect_height = RECTHEIGHT(*pRect);
 
 	// center of the rectangle
-	int rect_cx = pRect->left + rect_width/2;
-	int rect_cy = pRect->top  + rect_height/2;
+	float rect_cx = pRect->left + rect_width/2.0f;
+	float rect_cy = pRect->top  + rect_height/2.0f;
 
 	// zoom fActor needed to scale the Actor to fill the rectangle
 	float fNewZoomX = rect_width  / m_size.x;