After thinking about how the `evenodd` fill rule works, I pretty sure
we don't actually need to do this. If I'm right, I'll back it out, and
it will be in the history if we need it for some other purpose.
`iD.behavior.drag` is like `d3.behavior.drag`, with the following differences:
* The `origin` function is expected to return an [x, y] tuple rather than an
{x, y} object.
* The events are `start`, `move`, and `end`.
(https://github.com/mbostock/d3/issues/563)
* The `start` event is not dispatched until the first cursor movement occurs.
(https://github.com/mbostock/d3/pull/368)
* The `move` event has a `loc` and `dxdy` [x, y] tuple properties rather
than `x`, `y`, `dx`, and `dy` properties.
* The `end` event is not dispatched if no movement occurs.
* An `off` function is available that unbinds the drag's internal event handlers.
* Delegation is supported via the `delegate` function.
