ThumbnailSource

A WebVTT thumbnail file for an item (#433), as many web players publish: each cue's text names an image, relative to the file, and a region of it, sprite.jpg#xywh=0,0,160,90, for the cue's time. uri is where the file is, and io, when set, reads it instead, as SubtitleSource.io does. The images are read through the file's own reader, or else the player's network, with the item's headers for an image on the item's own server.