GL_SGIS_texture_filter4
http://oss.sgi.com/projects/ogl-sample/registry/SGIS/texture_filter4.txt
	void glGetTexFilterFuncSGIS (GLenum target, GLenum filter, GLfloat* weights)
	void glTexFilterFuncSGIS (GLenum target, GLenum filter, GLsizei n, const GLfloat* weights)