Fix sign of Box2.Size() and Box3.Size()

[gbenson]

Box2.Size() and Box3.Size() both subtracted the vectors in the wrong order, so the results were always negative.

e.g. a Box2 with points (-1, 4) and (5,-3) would have min=(-1,-3) and max=(5,4), so a Size() of (-6,-7) before this change.