WGL_NV_render_depth_texture
http://oss.sgi.com/projects/ogl-sample/registry/NV/render_depth_texture.txt
	WGL_NO_TEXTURE_ARB 0x2077
	WGL_BIND_TO_TEXTURE_DEPTH_NV 0x20A3
	WGL_BIND_TO_TEXTURE_RECTANGLE_DEPTH_NV 0x20A4
	WGL_DEPTH_TEXTURE_FORMAT_NV 0x20A5
	WGL_TEXTURE_DEPTH_COMPONENT_NV 0x20A6
	WGL_DEPTH_COMPONENT_NV 0x20A7