PositionAttitudeTransform, removed the equivialnt callbacks once found in these
transform classes.
Changed the NodeCallback class so its derived from osg::Object instead of
osg::Referenced to allow it to be saved out in the .osg format.
Added support for Update and Cull callbacks into the .osg file format.
Added support for AnimationPathCallback into the .osg file format.
added since the png loader itself doesn't not seem robust enough at detecting
non png files.
Added a tab in to the flt code to make it more obvious what is happening in
the flow of the code.
for settings options in osgDB::Registry, and added the paramter to all of the
reaader/writer plugins. The Options structure by default has an string attached
for packing basic options, however, it also can be subclassed to encapsulate
any users defined option data. In the later case both the client code *and*
the plugin need to be aware of subclass, the plugin will need to use
dynamic_cast<> to assertain its type.
