FeatureOverlay on ol.layer.Vector and ol.source.ImageVector

This change makes feature overlays work on vector layers and image vector sources. This has two benefits:

* Feature overlays now also work with vector content from ImageVector sources when a different renderer than canvas is used.
* Feature overlays can now be closely tied to a layer, allowing other content to be stacked on top of them

The use case that made me create this pull request is a support request to be able to mix and match dom and canvas rendering on a single map, and the missing piece was feature selection. This can now be achieved by drawing vector content on an image layer with vector image source and overlaying the selection overlay on that image layer.