add a demo for opengl. drawing a triangle.

 rerence:

 http://http.developer.nvidia.com/CgTutorial/cg_tutorial_chapter01.html

 http://www.opengl-tutorial.org/

 http://antongerdelan.net/opengl/index.html

 http://www.arcsynthesis.org/gltut/

 http://www.scratchapixel.com/

 http://duriansoftware.com/joe/An-intro-to-modern-OpenGL.-Chapter-1:-The-Graphics-Pipeline.html