function isAsyncSrc(src) {
  return typeof src === 'string' || Array.isArray(src) && typeof src[0] === 'string';
}
Check if a src is an async request.
if src is a string we're going to download an image
if src is an array of strings we're going to download cubemap images
@param {*} src The src from a TextureOptions
@returns {bool} true if src is async.
@private