Currently the vapoursynth video filter does not accept any argument for
passing arbitrary user data. This limits what the VS script can do.
Ideally, the vapoursynth filter has an user-data parameter that contain
string value. mpv passes that value to the VS script just like
container_fps and others. Once the VS script gets the data, it can do
all sorts of data extraction and transformation.
Another benefit is that instead of mpv always have to catch up to user
needs for this filter, with this users can just pass whatever needed
themselves, thus becomes more future-proof.
Fixes#14214
Crend King
fe709c986b