Add Z-index to layers

This commit allows setting a z-index property on the layers and uses
it in the canvas, WEBGL and DOM map renderers for ordering the layers
before rendering.

Default z-index is 0 for managed layers and 1000 for unmanaged ones.
It allows always on bottom, always on top and more complex layer layouts.

src/ol/renderer/webgl/webglmaprenderer.js

@@ -2,6 +2,7 @@
 
 goog.provide('ol.renderer.webgl.Map');
 
+goog.require('goog.array');
 goog.require('goog.asserts');
 goog.require('goog.dom');
 goog.require('goog.dom.TagName');
@@ -471,6 +472,8 @@ ol.renderer.webgl.Map.prototype.renderFrame = function(frameState) {
   /** @type {Array.<ol.layer.LayerState>} */
   var layerStatesToDraw = [];
   var layerStatesArray = frameState.layerStatesArray;
+  goog.array.stableSort(layerStatesArray, ol.renderer.Map.sortByZIndex);
+
   var viewResolution = frameState.viewState.resolution;
   var i, ii, layerRenderer, layerState;
   for (i = 0, ii = layerStatesArray.length; i < ii; ++i) {