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/maprenderer.js

@@ -375,3 +375,13 @@ ol.renderer.Map.prototype.scheduleRemoveUnusedLayerRenderers =
     }
   }
 };
+
+
+/**
+ * @param {ol.layer.LayerState} state1
+ * @param {ol.layer.LayerState} state2
+ * @return {number}
+ */
+ol.renderer.Map.sortByZIndex = function(state1, state2) {
+  return state1.zIndex - state2.zIndex;
+};