Add Bilateral prefilter

[LightArrowsEXE]

Bilateral filtering is useful for stuff like handling halos and the like, and since Prefilter is used by vsdehalo, it seems like a good call to add it here.

I propose two implementations:

1. Regular bilateral.

This is simply calling bilateral without doing anything special.

2. Double bilateral call, with one being passed to the second call as a ref clip.

This helps better preserve edges. See vsdehalo.bidehalo for reference. The idea is that you're doing two bilateral calls: the first call has a strong sigma/radius, and the second has a weaker sigma/radius. Ask zastin for more information, as he came up with this.

I think this is useful primarily for dehaloing operations, or for cases where you have very dirty edges and other prefilters affect those too much. It might also not be a bad idea to make the 1st option a default for vsdehalo.HQDeringmod, as it's generally a lot better than other options.

[Setsugennoao]

Added in 957131e