Scale StaticImage image to imageExtent

The imageSize property was only used to determine the scale. It's usage was
never documented nor tested nor shown in an example, so I removed it because I
considered its presence a bug.

src/ol/image.js

@@ -35,7 +35,7 @@ ol.Image = function(extent, resolution, pixelRatio, attributions, src,
 
   /**
    * @private
-   * @type {Image}
+   * @type {HTMLCanvasElement|Image|HTMLVideoElement}
    */
   this.image_ = new Image();
   if (crossOrigin) {
@@ -44,7 +44,7 @@ ol.Image = function(extent, resolution, pixelRatio, attributions, src,
 
   /**
    * @private
-   * @type {Object.<number, Image>}
+   * @type {Object.<number, (HTMLCanvasElement|Image|HTMLVideoElement)>}
    */
   this.imageByContext_ = {};
 
@@ -142,6 +142,14 @@ ol.Image.prototype.load = function() {
 };
 
 
+/**
+ * @param {HTMLCanvasElement|Image|HTMLVideoElement} image Image.
+ */
+ol.Image.prototype.setImage = function(image) {
+  this.image_ = image;
+};
+
+
 /**
  * Discards event handlers which listen for load completion or errors.
  *