Sort symbolizer groups by zIndex

Now rendering order can be controlled by setting the zIndex
symbolizer property.

src/ol/layer/vectorlayer.js

@@ -320,6 +320,7 @@ ol.layer.Vector.prototype.groupFeaturesBySymbolizerLiteral =
       }
     }
   }
+  featuresBySymbolizer.sort(this.sortByZIndex);
   return featuresBySymbolizer;
 };
 
@@ -454,6 +455,17 @@ ol.layer.Vector.prototype.setTemporary = function(temp) {
 };
 
 
+/**
+ * Sort function for `groupFeaturesBySymbolizerLiteral`.
+ * @param {Array} a 1st item for the sort comparison.
+ * @param {Array} b 2nd item for the sort comparison.
+ * @return {number} Comparison result.
+ */
+ol.layer.Vector.prototype.sortByZIndex = function(a, b) {
+  return a[1].zIndex - b[1].zIndex;
+};
+
+
 /**
  * @param {Array.<ol.Feature>} features Features.
  * @return {string} Feature info.