Configurable backbuffer order

Since we don't rely on the DOM order for layers, we also cannot rely on it for backbuffers. This change introduces z-index for backbuffer containers, and makes the backbuffer order configurable, e.g.

    var labels = new OpenLayers.Layer.WMS('Labels', 'http://my/wms', {layers: 'labels'}, {transitionEffect: 'map-resize'});
    var thematic = new OpenLayers.Layer.WMS('USA Population', 'http://my/wms', {layers: 'topp:states'});

The labels layer's backbuffer will be placed below the base layer, whereas the thematic layer's backbuffer will be placed right below the layer itself.