From Ravi Mathur, added extension checks for point sprite support.

From Robert Osfield, tweaks of the above to use osg::buffer_object and a local struct to store
initialized to help with multi-thread and out of order context usage.

include/osg/PointSprite

@@ -50,6 +50,8 @@ public:
 
 	virtual void apply(osg::State& state) const;
 
+	static bool isPointSpriteSupported(unsigned int context);
+
 protected:
 	virtual ~PointSprite( void ) {}
 };