HEIC: save extra preview image for large main image. HEIC do not support IPTC, so not preview tag can be used here. HEIC can store more than one version of an image, and especially a "thumbnail" can be assigned to a main image. Thumbnail is not a very smaller version of image (aka 256x256 for exif thumbnail), but just a reduced version always smaller than original. So we use the same rules than IPTC preview as we render a 1280x1280 version of image if main image size is greater than thumbnail.