add a new prepareData method to the interface

include/osgAnimation/RigTransform

@@ -33,6 +33,10 @@ namespace osgAnimation
 
         virtual void operator()(RigGeometry&) {}
 
+        /// to call manually when a skeleton is reacheable from the rig
+        /// in order to prepare technic data before rendering
+        virtual bool prepareData(RigGeometry&){}
+
     protected:
         virtual ~RigTransform() {}