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/canvas/canvasmaprenderer.js

@@ -2,6 +2,7 @@
 
 goog.provide('ol.renderer.canvas.Map');
 
+goog.require('goog.array');
 goog.require('goog.asserts');
 goog.require('goog.dom');
 goog.require('goog.style');
@@ -168,6 +169,8 @@ ol.renderer.canvas.Map.prototype.renderFrame = function(frameState) {
   this.dispatchComposeEvent_(ol.render.EventType.PRECOMPOSE, frameState);
 
   var layerStatesArray = frameState.layerStatesArray;
+  goog.array.stableSort(layerStatesArray, ol.renderer.Map.sortByZIndex);
+
   var viewResolution = frameState.viewState.resolution;
   var i, ii, layer, layerRenderer, layerState;
   for (i = 0, ii = layerStatesArray.length; i < ii; ++i) {