Add hit-detection images of symbols to separate atlas

This PR ...

* Adds the methods `getHitDetectionOrigin` and `getHitDetectionImageSize` to `ol.style.Image`.
* Adds a list of hit-detection atlases to the `AtlasManager`. If a `renderHitCallback` is provided, when adding an entry, the image is rendered into a hit-detection atlas image using that callback.
* Modifies the circle and regular-shape style, so that the hit-detection image is added to an atlas.
We use `goog.isNull` elsewhere in the library.