Added callbacks to allow users to modify the underlying functionality of

LOD, Billboard and Drawabel objects.

src/osg/Drawable.cpp

@@ -24,6 +24,16 @@ Drawable::Drawable()
 
 }
 
+Drawable::Drawable(const Drawable& drawable,const CopyOp& copyop):
+    Object(drawable,copyop),
+    _dstate(copyop(drawable._dstate.get())),
+    _supportsDisplayList(drawable._supportsDisplayList),
+    _useDisplayList(drawable._useDisplayList),
+    _globjList(drawable._globjList),
+    _bbox(drawable._bbox),
+    _bbox_computed(drawable._bbox_computed),
+    _drawCallback(drawable._drawCallback)
+{}
 
 Drawable::~Drawable()
 {