4K videos end up pixelated when the gallery is rendered

[fr33tux]

Hello,

4K videos are rendered with a very low quality when resized to 1280x720:
The original video: https://cloud.fr33tux.org/index.php/s/WtXxE4LSawJBf6F
The rendered result: https://pics.fr33tux.org/pointe_du_raz_2021/22.mp4.webm

Using Prosopopee 1.1.4 and f96fd7f, with ffmpeg:

settings:
  ffmpeg:
    binary: "ffmpeg"