Draw selected features only once

As discussed in https://github.com/openlayers/ol3/issues/1691#issuecomment-35521331, this adds a way for the select interaction to keep track of the layers that the selected features belong to. I know this is a lot of synchronization code right now, but it can be simplified once we decide that the select interaction is configured with a feature collection instead of a FeatureOverlay.

@elemoine: I know that the renderGeometryFunction is to go away, so I only made minimal changes related to that. All I needed was the feature as 2nd argument to get more context.

This depends on #1833.