Make Box::border_box a size, not a rect

[pcwalton]

It overlaps in confusing ways with Box::border_box. Box::border_box is the canonical source of this information. Right now, BaseFlow::position is mostly just used to store height and offset information that block flows need to perform block layout, but this can be moved into BlockFlow and/or split out from position as appropriate.

I think that height is the only thing that is really needed as part of BaseFlow, so that block flows know how tall their kids are without having to ask them to go to their box list. Maybe width too, for floats.

@SimonSapin You're probably interested in this, as you alluded to this as a source of confusion.

[pcwalton]

The title reflects what I think should be done now. Obviously I wasn't thinking straight when I wrote the above: the flows need to have position information so that parents can lay out their kids.

[dralley]

Is this still relevant?