This repo contains a homebrew demo for the 3DS the replicates the common retro game console effect of offsetting scanlines horizontally. It's done here through a geometry shader that when given an Nx16 sprite, makes 16 one-pixel-tall strips (2 triangles each). Each strip is then offset by indexing into an array uniform that contains a value for every y-value on the screen.
Contained here is the entirety of citro2d, hackily edited to work with this. it was done just for convenience - you could certainly make it play nicely with citro2d without having to edit it.
Please feel free to use this however you want!