facilitate the subclassing of Image providing interactive behaviours so as
used in the vnc interactive VncImage class.
osgViewer::InteractiveImageHandler provides an event handler that convertes osgGA
mouse and keyboard events into the coordinate frame of an image based on ray intersection with geometry in
the associated subgraph.
Changed the ordering of events processing in Viewer and CompositeViewer to allow
scene graph event handlers to take precidence over viewer event handlers and camera manipulators
70e1c60819