* update to use webp for thumbnails
* bump webp quality up to 40% from 12% (it's a bit different to jpeg quality setting)
* update to use yuva colorspace, and use thumbnail=n=10 to select frame
* fix missing comma in ffmpeg args
* add links to appropriate ffmpeg docs
* update tests
* add file size tests for thumbnails
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Co-authored-by: tobi <tobi.smethurst@protonmail.com>
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