ugoiras: encode .webm samples using VP9 instead of VP8.

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 after 4c652cf3e.
4c652cf3e tried 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/

app/logical/media_file/ugoira.rb

@@ -85,7 +85,10 @@ class MediaFile::Ugoira < MediaFile
       end
 
       ext = zipfile.first.name.match(/\.(\w{,4})$/)[1]
-      ffmpeg_out, status = Open3.capture2e("ffmpeg -i #{tmpdir}/images/%06d.#{ext} -codec:v libvpx -crf 12 -b:v 0 -an #{tmpdir}/tmp.webm")
+      ffmpeg_out, status = Open3.capture2e("ffmpeg -i #{tmpdir}/images/%06d.#{ext} -codec:v libvpx-vp9 -crf 12 -b:v 0 -an -threads 8 -tile-columns 2 -tile-rows 1 -row-mt 1 -pass 1 -f null /dev/null")
+      raise Error, "ffmpeg failed: #{ffmpeg_out}" unless status.success?
+
+      ffmpeg_out, status = Open3.capture2e("ffmpeg -i #{tmpdir}/images/%06d.#{ext} -codec:v libvpx-vp9 -crf 12 -b:v 0 -an -threads 8 -tile-columns 2 -tile-rows 1 -row-mt 1 -pass 2 #{tmpdir}/tmp.webm")
       raise Error, "ffmpeg failed: #{ffmpeg_out}" unless status.success?
 
       mkvmerge_out, status = Open3.capture2e("mkvmerge -o #{output_file.path} --webm --timecodes 0:#{tmpdir}/timecodes.tc #{tmpdir}/tmp.webm")