Add geometry primitives in flow-relative space

This is in preparation for the Big Layout Rewrite for CSS Writing Modes, which will come in a later pull request.

This is largely based on [Gecko’s `WritingModes.h`](http://dxr.mozilla.org/mozilla-central/source/layout/generic/WritingModes.h).

In the spec:
* Introduction / context: http://dev.w3.org/csswg/css-writing-modes/#text-flow
* Terminology: http://dev.w3.org/csswg/css-writing-modes/#abstract-box

r? @larsbergstrom