Feature Request: add adaptive grain filter

[Dan64]

Hello Selur,

It could be useful add in Hybrid the AdaptiveGrain filter described in this article: https://blog.kageru.moe/legacy/adaptivegrain.html

In summary the filter described aims to apply the optimal amount of grain to all scenes to prevent banding artifacts without having a significant impact on the perceived image quality or the required bitrate.

The filter can be downloaded here: https://git.kageru.moe/kageru/adaptivegr.../tag/0.3.1

Thanks