GL_NV_fence
http://oss.sgi.com/projects/ogl-sample/registry/NV/fence.txt
	GL_ALL_COMPLETED_NV 0x84F2
	GL_FENCE_STATUS_NV 0x84F3
	GL_FENCE_CONDITION_NV 0x84F4
	void glDeleteFencesNV (GLsizei n, const GLuint* fences)
	void glFinishFenceNV (GLuint fence)
	void glGenFencesNV (GLsizei n, GLuint* fences)
	void glGetFenceivNV (GLuint fence, GLenum pname, GLint* params)
	GLboolean glIsFenceNV (GLuint fence)
	void glSetFenceNV (GLuint fence, GLenum condition)
	GLboolean glTestFenceNV (GLuint fence)