Allow cache-only thumbnails for network files using `dump-cache`

[BanchouBoo]

dump-cache lets you dump a region of the cache to a video file. You can set the start and end to the same point in the video then extract the first frame from that output video.

You can check if a part of the video at a timestamp is cached by checking seekable-ranges in the demuxer-cache-state property.

This will be much faster than the current method for fetching network thumbnails.

[po5]

I was under the impression we'd be unable to know when the cache dump is done, but the manual says "Note that this command will finish only once dumping ends." so it might be possible without too much latency. I'm still worried it will lead to high memory usage, but I guess nothing can be as bad as what we're already doing.