…rties

… and converting them to flow-relative geometric values.

These values are almost always used to size and position a fragment within its containing block, so using the mode of the containing block seems more correct.

Note that the `writing-mode` and `direction` properties are disabled in Servo at the moment, so this PR by itself should have no effect: the writing mode of an element is always the same of that of its containing block since they’re both horizontal rtl.