Allow z index to be undefined

src/ol/replay/canvas/canvasbatchgroup.js

@@ -325,7 +325,7 @@ ol.replay.canvas.BatchGroup.prototype.finish = function() {
  * @inheritDoc
  */
 ol.replay.canvas.BatchGroup.prototype.getBatch = function(zIndex, batchType) {
-  var zIndexKey = zIndex.toString();
+  var zIndexKey = goog.isDef(zIndex) ? zIndex.toString() : '0';
   var batches = this.batchesByZIndex_[zIndexKey];
   if (!goog.isDef(batches)) {
     batches = {};

src/ol/replay/ibatchgroup.js

@@ -64,7 +64,7 @@ ol.replay.IBatchGroup.prototype.finish = function() {
 
 
 /**
- * @param {number} zIndex Z index.
+ * @param {number|undefined} zIndex Z index.
  * @param {ol.replay.BatchType} batchType Batch type.
  * @return {ol.replay.IBatch} Batch.
  */

src/ol/style.js

@@ -62,7 +62,7 @@ ol.style.stroke.equals = function(strokeStyle1, strokeStyle2) {
  * @typedef {{fill: ?ol.style.Fill,
  *            image: ?ol.style.Image,
  *            stroke: ?ol.style.Stroke,
- *            zIndex: number}}
+ *            zIndex: (number|undefined)}}
  */
 ol.style.Style;