Added osg::ShadeModel state attribute which encapsulates glShadeModel.

src/osg/ShadeModel.cpp

@@ -0,0 +1,19 @@
+#include <osg/GL>
+#include <osg/ShadeModel>
+
+using namespace osg;
+
+ShadeModel::ShadeModel()
+{
+    _mode = SMOOTH;
+}
+
+
+ShadeModel::~ShadeModel()
+{
+}
+
+void ShadeModel::apply(State&) const
+{
+    glShadeModel((GLenum)_mode);
+}