ImgPath variable if it is set, because it can't be set before the
OpenLayers.js file is loaded. Move ImgPath calculation into initialize(),
so that it is picked up correctly. Thanks, fvanderbiest! (Closes#1473)
git-svn-id: http://svn.openlayers.org/trunk/openlayers@6734 dc9f47b5-9b13-0410-9fdd-eb0c1a62fdaf