GL_SGIS_texture_filter4
http://www.opengl.org/registry/specs/SGIS/texture_filter4.txt
	void glGetTexFilterFuncSGIS (GLenum target, GLenum filter, GLfloat* weights)
	void glTexFilterFuncSGIS (GLenum target, GLenum filter, GLsizei n, const GLfloat* weights)
