Async ATF support in ImageLoader

[esidegallery]

I've noticed that although there is an asyncTextureUpload property, it doesn't apply to ATF files (this._texture.root.uploadAtfData(rawData) and Texture.fromAtfData(rawData, this._scaleFactor) are not defining async functions/flags). Although ATF uploading is a lot faster than BMP, I'm still noticing a small hit while this happens during scrolling.