box-layout: remove use of ClutterAlpha
box-layout: remove use of ClutterAlpha

clutter-project/mx,jonnylamb/mx,jonnylamb/mx,pwithnall/mx,jonnylamb/mx,clutter-project/mx,clutter-project/mx,jonnylamb/mx,pwithnall/mx,pwithnall/mx,clutter-project/mx,pwithnall/mx