Adds a --layer option to override the Z Index the shader is rendered at

[minimpoun]

I've been using Papertoy for my wallpaper, but I wanted to also use it for my screen saver. However, papertoy currently only renders to the background layer.

This change adds a cmd arg to specify which layer the shader is rendered to. I use this in my screen saver script that's ran by hypridle like so: exec papertoy "$shader" --layer top

Still defaults to using the background layer, so functionality remains unchanged if you don't pass a --layer arg.