Abandoned. https://github.com/aaronfranke/GraphicsEditor is exaclty what I planned this to be, even if I end up coninuing this I'd switch to using that as a base.
My goal with this project is to take Pixelorama's code, simplify it as much as possible, and make it an editor plugin, to have an in-godot texture/image editor.
If it turns out well I'll probably turn it into a c++ engine module eventually.