GL_NV_occlusion_query
http://oss.sgi.com/projects/ogl-sample/registry/NV/occlusion_query.txt
	GL_PIXEL_COUNTER_BITS_NV 0x8864
	GL_CURRENT_OCCLUSION_QUERY_ID_NV 0x8865
	GL_PIXEL_COUNT_NV 0x8866
	GL_PIXEL_COUNT_AVAILABLE_NV 0x8867
	void glBeginOcclusionQueryNV (GLuint id)
	void glDeleteOcclusionQueriesNV (GLsizei n, const GLuint* ids)
	void glEndOcclusionQueryNV (void)
	void glGenOcclusionQueriesNV (GLsizei n, GLuint* ids)
	void glGetOcclusionQueryivNV (GLuint id, GLenum pname, GLint* params)
	void glGetOcclusionQueryuivNV (GLuint id, GLenum pname, GLuint* params)
	GLboolean glIsOcclusionQueryNV (GLuint id)