function loadAndUseImage(obj, crossOrigin, callback) {
  if (isTexImageSource(obj)) {
    setTimeout(function () {
      callback(null, obj);
    });
    return obj;
  }

  return loadImage(obj, crossOrigin, callback);
}
if obj is an TexImageSource then just
uses it otherwise if obj is a string
then load it first.

@param {string|TexImageSource} obj
@param {string} crossOrigin
@param {function(err, img)} [callback] a callback that's passed an error and the image. The error will be non-null
    if there was an error
@private