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.
I think the compiler knows because `this.geometryFunction_` is defined [here](https://github.com/ahocevar/ol3/blob/style-geometry/src/ol/style/style.js#L35).