Do you plan on making this a library?
Also is there a way to remove all viewable sprites in the atlas? It'll be useful for fighting games that need a lot of sprites for animation.
What do you mean? There is a clean button on the sprites list to remove all sprites from the atlas.
There can be sprites that have a red border around them that are not on the atlas, is there a way to automatically delete all the viewable sprites (ones without the red border)?
Actually most of the functionality of this tool is directly available in raylib. You can use raylib.