Added realize() and isRealized() methods to osg::GraphicsContext.

Added osgcamera example that uses osg::GraphicsContext to create the required
window for rendering too, will eventually use osg::CameraNode to replace usage
of osgUtil::SceneView.

include/osgProducer/GraphicsContextImplementation

@@ -36,7 +36,6 @@ class OSGPRODUCER_EXPORT GraphicsContextImplementation : public osg::GraphicsCon
         GraphicsContextImplementation(Producer::RenderSurface* rs);
 
         /** Return true of graphics context is realized.*/
-        bool isRealized() { return _rs.valid() && _rs->isRealized(); }
 
         /** Return the RenderSurface that implements the graphics context.*/
         Producer::RenderSurface* getRenderSurface() { return _rs.get(); }
@@ -45,6 +44,12 @@ class OSGPRODUCER_EXPORT GraphicsContextImplementation : public osg::GraphicsCon
         const Producer::RenderSurface* getRenderSurface() const { return _rs.get(); }
 
 
+        /** Realise the GraphicsContext.*/
+        virtual bool realize();
+
+        /** Return true if the graphics context has been realised and is ready to use.*/
+        virtual bool isRealized() const { return _rs.valid() && _rs->isRealized(); }
+
         /** Release the graphics context.*/
         virtual void release();