Update Vector.js to support setOpacity for vector layers within vector RootContainer

There's forgotten simple patch in trac http://trac.osgeo.org/openlayers/ticket/3608 by Christopher Schmidt, which I'm using for over year and Heron community uses it too https://code.google.com/p/geoext-viewer/source/browse/trunk/heron/lib/override-openlayers.js. It would be fine to merge it.