Allow styles to override feature geometries

With this change, application developers are able to define styles that
render a different geometry than the feature geometry. This can e.g. be
used to render an interior point of a polygon instead of the polygon, or
to render symbols like arrows along lines.
Oh, I see your point @tschaub. The function's body should be
```js
var result = feature.get(geometry);
return goog.isDef(result) ? result : null;
```
Really strange that the compiler does not catch this.