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/Tile.js

@@ -326,6 +326,18 @@ class TileSource extends Source {
    */
   useTile(z, x, y, projection) {}
 
+  /**
+   * 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. If undefined,
+   * the decision is left to the renderer.
+   * @return {number|undefined} Prefered zDirection for source resolution selection.
+   */
+  getZDirection() {
+    return undefined;
+  }
+
 }