code: function (renderer) {
          var _gl = renderer.gl;
          for (var attachment in this._textures) {
              var obj = this._textures[attachment];
              if (obj) {
                  var texture = obj.texture;
                  // FIXME some texture format can't generate mipmap
                  if (!texture.NPOT && texture.useMipmap
                      && texture.minFilter === Texture.LINEAR_MIPMAP_LINEAR) {
                      var target = texture.textureType === 'textureCube' ? glenum.TEXTURE_CUBE_MAP : glenum.TEXTURE_2D;
                      _gl.bindTexture(target, texture.getWebGLTexture(renderer));
                      _gl.generateMipmap(target);
                      _gl.bindTexture(target, null);
                  }
              }
          }
      }
docs: Because the data of texture is changed over time, Here update the mipmaps of texture each time after rendered;