Use default style function in FeatureOverlay
Fix the pointer index

It has to point to the end of the current block, not to the
beginning of the next block. See L581.
Merge pull request #1827 from elemoine/featureoverlay

Use default style function in FeatureOverlay
Merge pull request #1833 from ahocevar/hit-detection-pointer

Fix the pointer index
Call renderGeometryFunction with opaque data object as 2nd arg
Make FeatureOverlay an ol.Observable and trigger change events

This allows us to get notified when e.g. the overlay's features
collection changes.
Do not render selected features on the original layer

By keeping track of the layers of selected features, we can
push a renderGeometryFunction to a layer so selected features
are no longer rendered. This also covers features that are
programmatically added to the selection overlay.
No need to monitor feature collection changes on the overlay

As @elemoine points out correctly, the FeatureOverlay is private
to the Select interaction, so it cannot be accessed by the
application to change its feature collection.