Made the drawable::compile(State&) method const and the DisplayListVisitor to

compile even when dislay list is off.  This has been done to allow subclasses
of Drawable handle their own compile implementation, such as used by text.

src/osg/Drawable.cpp

@@ -85,7 +85,7 @@ void Drawable::dirtyBound()
     }
 }
 
-void Drawable::compile(State& state)
+void Drawable::compile(State& state) const
 {
     if (!_useDisplayList) return;