[Feature] Channel selector

[dishmint]

    Could be interesting to have agents for each color channel.  imagine sending out a green agent, where the seed image is preserved, but only the green channel is affected.

Originally posted by @dishmint in #42 (comment)

Choose which channels to apply filtering to. r,g,b or a.

[dishmint]

Or, grayscale the image and put that gsc value in one of the channels.

to keep the original image in one of the channels, and use the others to observe the propagation of the filter.

[dishmint]

An ArcaneProcess takes the pixel and applies the filter to all channels.
Might need to refactor ArcaneProcess that takes in a pixel, then internally can decide how to process the channels. Not sure how it would work with built-in filters like BLUR, maybe this is where mask() would be useful?

[dishmint]

What I really want is an overlay, so I can see the filter propagation on top of the original image.

Otherwise showing the filter on a single channel wouldn't actually demonstrate the affect on the whole image.