Give styles an exportable getGeometry method

I think this makes the use of the geometry functions more natural.  E.g.

```js
var geometry = style.getGeometry(feature);
```

instead of

```js
var geometry = style.getGeometryFunction().call(style, feature);
```

I've also made the `style.getGeometry()` method exportable so it can be used outside the library (as with the other style methods).

Though we don't depend on it internally, as the tests demonstrate, `style.getGeometry()` calls any custom geometry getter with `this` set to `style` by default (by nature of the syntax, not with an explicit call to `call`).  Users may provide a getter function that has a different `this` object bound.