layout: Implement `outline` per CSS 2.1 § 18.4.

`invert` is not yet supported.

Objects that get layers will not yet display outlines properly. This is
because our overflow calculation doesn't take styles into account and
because layers are always anchored to the top left of the border box.
Since fixing this is work that is not related to outline *per se* I'm
leaving that to a followup and making a note in the code.
auto merge of #4299 : pcwalton/servo/outline, r=mbrubeck

`invert` is not yet supported.

Objects that get layers will not yet display outlines properly. This is
because our overflow calculation doesn't take styles into account and
because layers are always anchored to the top left of the border box.
Since fixing this is work that is not related to outline *per se* I'm
leaving that to a followup and making a note in the code.

r? @SimonSapin