Descale - Resamplers to undo upscaling

[lextra2]

https://github.com/Jaded-Encoding-Thaumaturgy/vapoursynth-descale?tab=readme-ov-file#custom-kernels

I feel like such a feature would be very helpful to allow users to undo upscaling used by some Blurays, older animes, for example, which wasn't mastered in 1920x1080.

In such instances it is often the case that bilinear or bicubic was used, which could then be undone by the user and upscaled with something sharper like lanczos or ML-based scalers.