add new vector style property "graphicOpacity"

enabling mixing non-opaque vector geometries with opaque external graphics on the same vector layer
(closes #873)


git-svn-id: http://svn.openlayers.org/trunk/openlayers@4114 dc9f47b5-9b13-0410-9fdd-eb0c1a62fdaf

lib/OpenLayers/Feature/Vector.js

@@ -256,6 +256,13 @@ OpenLayers.Feature.Vector = OpenLayers.Class(OpenLayers.Feature, {
  *  - hoverPointRadius: 1,
  *  - hoverPointUnit: "%",
  *  - pointerEvents: "visiblePainted"
+ *
+ * Other style properties that have no default values:
+ *
+ *  - externalGraphic,
+ *  - graphicWidth,
+ *  - graphicHeight,
+ *  - graphicOpacity
  */ 
 OpenLayers.Feature.Vector.style = {
     'default': {