per scene view. The user can attach a NodeVisitor to do init for them, or
leave it to the default which is to use the osgUtil::DisplayListVisitor
which compiles all display lists and texture objects. The init traversal
is called automatically by the first call to either app() or cull(), so
should not be called by user code during initialization. This ensures
that a valid graphics context has been established before OpenGL is initialized.
osgUtil::DisplayListVisitor has also been updated to use a bit mask for options, and the addition of
compilation of texture objects (via StateAttribute::compile) has also been
added.
