GL_EXT_stencil_two_side
http://oss.sgi.com/projects/ogl-sample/registry/EXT/stencil_two_side.txt
	GL_STENCIL_TEST_TWO_SIDE_EXT 0x8910
	GL_ACTIVE_STENCIL_FACE_EXT 0x8911
	void glActiveStencilFaceEXT (GLenum face)