Add a zDirection option for Zoomify

Via zDirection, the renderer's selection of a tile source's resolution can be
controlled if the view's resolution does not match any tile source resolution.

Currently the zDirection option is only available for Zoomify. The comment
https://github.com/openlayers/openlayers/issues/9343#issuecomment-474341033
describes why caution is advised when using zDirection for other sources.

ol/source/Tile~TileSource#getZDirection has been added to avoid having the
getZDirection method solely in the Zoomify source - just to document the
possibility.

src/ol/source/Zoomify.js

@@ -111,6 +111,10 @@ export class CustomTile extends ImageTile {
  * @property {number} [transition] Duration of the opacity transition for rendering.
  * To disable the opacity transition, pass `transition: 0`.
  * @property {number} [tileSize=256] Tile size. Same tile size is used for all zoom levels.
+ * @property {number} [zDirection] Indicate which resolution should be used
+ * by a renderer if the views resolution does not match any resolution of the tile source.
+ * If 0, the nearest resolution will be used. If 1, the nearest lower resolution
+ * will be used. If -1, the nearest higher resolution will be used.
  */
 
 
@@ -253,6 +257,19 @@ class Zoomify extends TileImage {
       transition: options.transition
     });
 
+    /**
+     * @protected
+     * @type {number|undefined}
+     */
+    this.zDirection = options.zDirection;
+
+  }
+
+  /**
+   * @inheritDoc
+   */
+  getZDirection() {
+    return this.zDirection;
   }
 
 }