box-layout: fix computation of step increment with horizontal box
box-layout: fix computation of step increment with horizontal box

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