Make zDirection configurable on tile source

src/ol/source/XYZ.js

@@ -41,6 +41,10 @@ import {createXYZ, extentFromProjection} from '../tilegrid.js';
  * @property {boolean} [wrapX=true] Whether to wrap the world horizontally.
  * @property {number} [transition] Duration of the opacity transition for rendering.
  * To disable the opacity transition, pass `transition: 0`.
+ * @property {number} [zDirection=0] Indicate which resolution should be used
+ * by a renderer if the view 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.
  */
 
 
@@ -94,7 +98,8 @@ class XYZ extends TileImage {
       urls: options.urls,
       wrapX: options.wrapX !== undefined ? options.wrapX : true,
       transition: options.transition,
-      attributionsCollapsible: options.attributionsCollapsible
+      attributionsCollapsible: options.attributionsCollapsible,
+      zDirection: options.zDirection
     });
 
   }