Thumbnail quality improvement

[CXwudi]

• I'm reporting a broken site support issue
• I've verified that I'm running youtube-dl version 2020.12.14
• 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 bug reports including closed ones
• I've read bugs section in FAQ

#5 was coded by me to try to improve the thumbnail quality but now it's failed again. Good thing is that with the new niconico api-data schema, the best quality thumbnail (720p) can be easily obtained through api_data['video']['thumbnail']['ogp'], this should work doesn't matter if it is a normal video or an HLS encrypted videos, also doesn't matter if user has login or not.

To better understand why, here is the portion of thumbnails information in api_data for sm38140033, from top to bottom are thumbnails from worst to best qualities.

      "thumbnail":{
         "url":"https://nicovideo.cdn.nimg.jp/thumbnails/38140033/38140033.82394287",
         "middleUrl":"https://nicovideo.cdn.nimg.jp/thumbnails/38140033/38140033.82394287.M",
         "largeUrl":"https://nicovideo.cdn.nimg.jp/thumbnails/38140033/38140033.82394287.L",
         "player":"https://img.cdn.nimg.jp/s/nicovideo/thumbnails/38140033/38140033.82394287.original/a960x540l?key=52a190060cf17d8280e439793c5352b2e2458abab080d187750f975bbd2126b5",
         "ogp":"https://img.cdn.nimg.jp/s/nicovideo/thumbnails/38140033/38140033.82394287.original/r1280x720l?key=85e590129f856ca5d37e61562bd4b20a3c9e4bba34c87f8b4ea07404ecc93fbc"
      },