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Switch the codec for .webm samples from VP8 to VP9. All modern browsers support VP9 (Safari was the last to add support in ~2020), so it should be safe to provide only VP9 .webms without a fallback. VP9 lets us use two-pass encoding, which should offer better compression. Fixes ugoira samples still having poor quality even after4c652cf3e.4c652cf3etried to remove the max bitrate limit by setting `-b:v 0`, but this only worked in FFmpeg 4.2. In production Danbooru uses FFmpeg 4.4, and apparently in 4.4 `-b:v 0` means "use the default max bitrate of 256kb/s" instead of "no bitrate limit". https://trac.ffmpeg.org/wiki/Encode/VP9 https://developers.google.com/media/vp9/bitrate-modes https://developers.google.com/media/vp9/settings/vod http://wiki.webmproject.org/ffmpeg/vp9-encoding-guide https://www.reddit.com/r/AV1/comments/k7colv/encoder_tuning_part_1_tuning_libvpxvp9_be_more/