Type-related changes to ol.style.Image

[elemoine]

This PR includes a number of type-related changes to ol.style.Image, ol.style.Icon and ol.style.Circle.

1. Mark ol.style.Image an @api annotation. This makes ol.style.Image exportable, making it possible to do imageStyle instanceof ol.style.Image in the application code.

2. Mark ol.style.Image#getImage with an @api annotation. This makes this method show up in the ol-externs.js externs file, which is necessary for application code that looks like this:

   /**
   * @param {ol.style.Image} imageStyle Image style.
   */
   function function1(imageStyle) {
     var image = imageStyle.getImage();
     function2(image);
   }
   
   /**
   * @param {HTMLCanvasElement|HTMLVideoElement|Image} image Image element.
   */
   function function2(image) {
    …
   }

   We have to have the signature of ol.style.Image#getImage in the externs file.

3. Give ol.style.Circle#getImage and ol.style.Icon#getImage more specific return types.

Please review.

[gberaudo]

Looks good.

[fredj]

LGTM