More power to the renderGeometryFunction

The geometry passed to the `renderGeometryFunction` is the simplified geometry, not the original feature geometry. With the feature that the geometry belongs to as 2nd argument, application developers can e.g. use the `renderGeometryFunction` to avoid rendering of features that are on a selection overlay.

The one thing I was not sure about when creating this pull request was whether there is a planned future use of the opaque data object that was passed to `ol.renderer.vector.renderFeature`. The only object that was ever passed to that function was an `ol.Feature`, so I considered that `data` argument redundant and removed it in the process of changing the type of that object from `{Object}` to `{ol.Feature}`.