Disable video chapter thumbnail generation per library

[jasonlp]

Based on the discussion here, it seems that video chapter thumbnail generation is an all-or-nothing toggle. Would like to have the ability to turn this off on a per-library basis, like we can do with video preview thumbnails.

Turn off video chapter thumbnails for specific library?

Chapter thumbnails are not the same as video preview thumbnails. The former only produce one picture for every chapter marker, while the latter produce a picture every 5 seconds (by default). So the amount of storage required for chapter thumbnails is quite negligible (unless you have files with hundreds of chapters).