Backbuffer needs z-index

Since we don't rely on the DOM order for layers, we also cannot rely on it
for backbuffers. This change simply adds a sensible z-index to the
backbuffer container.

lib/OpenLayers/Layer/Grid.js

@@ -705,6 +705,8 @@ OpenLayers.Layer.Grid = OpenLayers.Class(OpenLayers.Layer.HTTPRequest, {
             backBuffer.id = this.div.id + '_bb';
             backBuffer.className = 'olBackBuffer';
             backBuffer.style.position = 'absolute';
+            backBuffer.style.zIndex = this.map.Z_INDEX_BASE.BaseLayer -
+                    (this.map.getNumLayers() - this.map.getLayerIndex(this));
             for(var i=0, lenI=this.grid.length; i<lenI; i++) {
                 for(var j=0, lenJ=this.grid[i].length; j<lenJ; j++) {
                     var tile = this.grid[i][j],