I think you can import sprites from another editor (like Aseprite, Paint.net, or if you really want, MS Paint itself) and likewise you can just edit the images from the project's save directory itself, and it updates in Gamemaker, doesn't it? (Just something to help out, but considering your work on shaders and how beautiful the demo looks to begin with, I doubt you need this little tip. Otherwise, I agree that Gamemaker's sprite editor is bad. (Good to at least "have," but otherwise I've never really touched it and much prefer other image editors because I'm used to them anyways.)
Hey, thanks for the comment on the game / shaders, I try to do my best.
Yeah, I know you can import sprites and I'm already using Aseprite for all the graphics.
Problem is that I'm already using sprite sheets in Aseprite and importing parts of an image is so annoying in the GM sprite editor, like the copy/paste thing for example. (which apparently is being 'fixed' in a new release)
I don't know how to explain it really, I just do not like anything related to the GM UI, it just all feels so bad to use..
So I try to avoid using it as much as possible and if there was a 'code only' version of GM, I'd likely be using that.
Anyway, the sprite editor is basically dead to me right now as I'm going all in on my own sprite system, which is better in the long run since I can use it for future projects as well and never have to deal with GM sprites again.