Added SceneView::Options enum enetries APPLY_GLOBAL_DEFAULTS and CLEAR_GLOBAL_STATESET to control whether a _globalStateSet->clear() and _globalStateSet->setGlobalDefaults() should be called.

osgViewer::Renderer doesn't use these enum settings so now no longer has a calls StateSet::clear() or StateSet::setGlobalDefaults() on the osg::Camera's StateSet.  Previously these were being
called and breaking the ability to attached state to Camera's StateSet.