function loadSide(index) {
      self.loadTextureSource(srcs[index], function (source) {
        cube.images[index] = source.data;
        loaded++;
        if (loaded === 6) {
          cube.needsUpdate = true;
          cb(cube);
        }
      });
    }
Load the six individual sides and construct a cube texture, then call back.

@param {Array<string|Element>} srcs - Array of six texture URLs or elements.
@param {function} cb - Callback to pass cube texture to.