GL_EXT_depth_bounds_test
http://www.nvidia.com/dev_content/nvopenglspecs/GL_EXT_depth_bounds_test.txt
	GL_DEPTH_BOUNDS_TEST_EXT 0x8890
	GL_DEPTH_BOUNDS_EXT 0x8891
	void glDepthBoundsEXT (GLclampd zmin, GLclampd zmax)