Because the corner points are duplicated, I don't need to do any averaging.  When the current position and the next position are the same point, it sets the `recomputeNormal` flag to true, signifying that the normal needs to be recomputed for the next position because we've reached a new wall.  This is also so it doesn't recompute the normal for every position, because all points on the same wall will have the same normal.