Attempt to fix broken DLL exports from osgDB
Attempt to fix broken DLL exports from osgDB

Only export the osgDB method implementations, instead of the entire
class, and hence avoid exporting symbols from the base class, which
then conflict with other compilation units when linking.

This avoids the need for /FORCE:MULTIPLE linker option with MSVC.
scrawl/osg,marchelbling/osg,marchelbling/osg,jtorresfabra/osg,openscenegraph/osg,openscenegraph/osg,tomasthoresen/osg,jtorresfabra/osg,openscenegraph/osg,tomasthoresen/osg,scrawl/osg,marchelbling/osg,scrawl/osg,openscenegraph/osg,tomasthoresen/osg,scrawl/osg,tomasthoresen/osg,marchelbling/osg,jtorresfabra/osg,jtorresfabra/osg