[osg-submissions] Fixes for VRML2 plugin

[Robert Osfield]

There are a few similarities between the .osg and .vrml so I wonder if
there might be a way of leveraging osgDB's parser support.  As part
the VPB work I added extra help classes and macro's that make putting
together parsing code easier, perhaps these might help.

X3D is XML so would need a very different parser, libXML2 is something
I've used in the past to good effect.

Robert.