[osg-submissions] osgconv --compressed not working under MSVC

[Robert Osfield]

Hi Luigi,

Rather than introduce a dumy osgViewer::Viewer object into an app that
doesn't need a viewer, I have changed the checking of the graphics
context so it does a check to see if the created context is a
osgViewer::GraphicWindow, the code then reported where its created a
graphics window or a pbuffer.  This should force the compiler to link
in osgViewer and the associated GraphcisWindow* implementation.

Could you do a svn update and let me know if osgconv --compressed now
works.  If it does I'll change osgdem to work like this too.

Cheers,
Robert.