We'll consider that as using existing assets you own the rights to. There's nothing stopping you from copying the shader line-for-line anyways, so you might as well just use it. As long as they weren't made specifically for this jam, we'll see it as your in-house library of game assets. Keep in mind that it could be hard to make pre-existing assets work with the theme of the jam, so trying to force them into the project could backfire! At the end of the day we just want to see some cool projects made!