Add ol.source.ImageArcGISRest (#3880)

* Add ol.source.ImageArcGISRest

ol.source.ImageArcGISRest is added so a single image can be retrieved
which is required when labelling is applied to the underlying map
service.
it's based on the approach used in ol.source.ImageWMS

* updating to remove as many good references as possible
changing so js passes lint checks

* fixing up so build and tests run

* putting back what was lost in the merge

* adding in an example
tidying up

externs/olx.js

@@ -3094,7 +3094,6 @@ olx.interaction.SelectOptions.prototype.toggleCondition;
  */
 olx.interaction.SelectOptions.prototype.multi;
 
-
 /**
  * Collection where the interaction will place selected features. Optional. If
  * not set the interaction will create a collection. In any case the collection
@@ -3105,7 +3104,6 @@ olx.interaction.SelectOptions.prototype.multi;
  */
 olx.interaction.SelectOptions.prototype.features;
 
-
 /**
  * A function that takes an {@link ol.Feature} and an {@link ol.layer.Layer} and
  * returns `true` if the feature may be selected or `false` otherwise.
@@ -4808,6 +4806,105 @@ olx.source.OSMOptions.prototype.url;
 olx.source.OSMOptions.prototype.wrapX;
 
 
+/**
+ * @typedef {{attributions: (Array.<ol.Attribution>|undefined),
+ *     crossOrigin: (null|string|undefined),
+ *     logo: (string|olx.LogoOptions|undefined),
+ *     imageLoadFunction: (ol.ImageLoadFunctionType|undefined),
+ *     params: Object.<string,*>,
+ *     projection: ol.proj.ProjectionLike,
+ *     ratio: (number|undefined),
+ *     resolutions: (Array.<number>|undefined),
+ *     url: (string|undefined)}}
+ * @api
+ */
+olx.source.ImageArcGISRestOptions;
+
+
+/**
+ * Attributions.
+ * @type {Array.<ol.Attribution>|undefined}
+ * @api
+ */
+olx.source.ImageArcGISRestOptions.prototype.attributions;
+
+
+/**
+ * The `crossOrigin` attribute for loaded images.  Note that you must provide a
+ * `crossOrigin` value if you are using the WebGL renderer or if you want to
+ * access pixel data with the Canvas renderer.  See
+ * {@link https://developer.mozilla.org/en-US/docs/Web/HTML/CORS_enabled_image}
+ * for more detail.
+ * @type {null|string|undefined}
+ * @api
+ */
+olx.source.ImageArcGISRestOptions.prototype.crossOrigin;
+
+
+/**
+ * Logo.
+ * @type {string|olx.LogoOptions|undefined}
+ * @api
+ */
+olx.source.ImageArcGISRestOptions.prototype.logo;
+
+
+/**
+ * Optional function to load an image given a URL.
+ * @type {ol.ImageLoadFunctionType|undefined}
+ * @api
+ */
+olx.source.ImageArcGISRestOptions.prototype.imageLoadFunction;
+
+
+/**
+ * ArcGIS Rest parameters. This field is optional. Service defaults will be
+ * used for any fields not specified. `FORMAT` is `PNG32` by default. `F` is `IMAGE` by
+ * default. `TRANSPARENT` is `true` by default.  `BBOX, `SIZE`, `BBOXSR`,
+ * and `IMAGESR` will be set dynamically. Set `LAYERS` to
+ * override the default service layer visibility. See
+ * {@link http://resources.arcgis.com/en/help/arcgis-rest-api/index.html#/Export_Map/02r3000000v7000000/}
+ * for further reference.
+ * @type {Object.<string,*>|undefined}
+ * @api
+ */
+olx.source.ImageArcGISRestOptions.prototype.params;
+
+
+/**
+ * Projection.
+ * @type {ol.proj.ProjectionLike}
+ * @api
+ */
+olx.source.ImageArcGISRestOptions.prototype.projection;
+
+
+/**
+ * Ratio. `1` means image requests are the size of the map viewport, `2` means
+ * twice the size of the map viewport, and so on. Default is `1.5`.
+ * @type {number|undefined}
+ * @api
+ */
+olx.source.ImageArcGISRestOptions.prototype.ratio;
+
+
+/**
+ * Resolutions. If specified, requests will be made for these resolutions only.
+ * @type {Array.<number>|undefined}
+ * @api
+ */
+olx.source.ImageArcGISRestOptions.prototype.resolutions;
+
+
+/**
+ * ArcGIS Rest service URL for a Map Service or Image Service. The
+ * url should include /MapServer or /ImageServer.
+ * @type {string|undefined}
+ * @api
+ */
+olx.source.ImageArcGISRestOptions.prototype.url;
+
+
 /**
  * @typedef {{attributions: (ol.AttributionLike|undefined),
  *     canvasFunction: ol.CanvasFunctionType,