thumbnailAt

The seek bar picture for position of the item that plays, or null when the item carries no pictures or none stands for that position (#433). The pictures come from the item's MediaItem.thumbnails file, or else from the stream itself, a DASH thumbnail set or an HLS image playlist, which Tracks.thumbnails lists. An image downloads only when it is asked for, and a few recent ones are kept, so asking at every step of a scrub costs one download for each image. The picture's times count from the item's start, as position does.