Supress thumbnail generation

While browsing for a file to open, VLC automatically tries to generate thumbnails for video files. This happens even if nothing is checked in "Media library folders" and if "Auto rescan" and "Playback history" are disabled. This behavior is new for 3.1.x and does not happen in 3.0.x, which always displayed a generic clapboard icon for all videos.

(Automatic thumbnail generation is potentially a privacy concern – plus it can potentially reveal spoilers, and in the case of a folder containing many similar videos, it doesn't help to distinguish things anyway.)