Setting opacity on the last child.

This is required for IFrame tiles, so we don't set the opacity on the event pane. To make this work with vector layers, the defs node needs to be inserted before the root group element instead of after it.

lib/OpenLayers/Renderer/SVG.js

@@ -470,7 +470,7 @@ OpenLayers.Renderer.SVG = OpenLayers.Class(OpenLayers.Renderer.Elements, {
      */
     createDefs: function() {
         var defs = this.nodeFactory(this.container.id + "_defs", "defs");
-        this.rendererRoot.appendChild(defs);
+        this.rendererRoot.insertBefore(defs, this.root);
         return defs;
     },