[twitch] [rechat] Twitch subtitles download only partially

[burtek]

Checklist

• I'm reporting a broken site support
• I've verified that I'm running youtube-dl version 2020.06.16.1
• I've checked that all provided URLs are alive and playable in a browser
• I've checked that all URLs and arguments with special characters are properly quoted or escaped
• I've searched the bugtracker for similar issues including closed ones

Verbose log

PS P:\> youtube-dl --version
2020.06.16.1
PS P:\> youtube-dl --skip-download --write-sub https://www.twitch.tv/videos/656610036 --verbose
[debug] System config: []
[debug] User config: ['-f', 'best', '--no-check-certificate']
[debug] Custom config: []
[debug] Command-line args: ['--skip-download', '--write-sub', 'https://www.twitch.tv/videos/656610036', '--verbose']
[debug] Encodings: locale cp1250, fs mbcs, out cp852, pref cp1250
[debug] youtube-dl version 2020.06.16.1
[debug] Python version 3.4.4 (CPython) - Windows-10-10.0.19041
[debug] exe versions: ffmpeg 3.3.1, ffprobe 3.3.1
[debug] Proxy map: {}
[twitch:vod] 656610036: Downloading vod info JSON
[twitch:vod] 656610036: Downloading vod access token
[twitch:vod] 656610036: Downloading m3u8 information
[info] Writing video subtitles to: Boundless! _ SMP SATURDAY! -v656610036.rechat.json

Description

Twitch rechat is now chunked into multiple files, same as video. youtube-dl downloads only first JSON, which ends with _next field, which is base-64 encoded JSON with next rechat JSON params:

eyJpZCI6IjBkOWNmZWNhLWI5MDAtNGFjYy1hODRmLTBjZmY5MjVlMmVkZSIsImhrIjoiYnJvYWRjYXN0OjM4NzkwMDI4MDE2Iiwic2siOiJBQUFBcU5BTXQwQVdHbGpJSGNSUVFBIn0f decodes here to {"id":"0d9cfeca-b900-4acc-a84f-0cff925e2ede","hk":"broadcast:38790028016","sk":"AAAAqNAMt0AWGljIHcRQQA"}

I think youtube-dl should follow _next fields, download whole rechat and merge it into single JSON.

P.S.: Really appreciate all work put into youtube-dl, it's a wonderful tool. Thank you for doing it :)