control whether or not to render a stroke, fill and graphic.
* added a defaultsPerSymbolizer property to OpenLayers.Style to allow
for extending incomplete symbolizers with defaults for stroke, fill or
graphic. This also makes Format.SLD read/write round trips possible
without modifying empty or incomplete <Stroke/>, <Fill/> and <Graphic/>
constructs. r=tschaub (closes#1876)
git-svn-id: http://svn.openlayers.org/trunk/openlayers@9278 dc9f47b5-9b13-0410-9fdd-eb0c1a62fdaf