[Navarro Parker]"I vaguely remember a tutorial doing it manually a 1 pixel mask or something like that."
To do this without third-party effects, I'd duplicate then precomp the image in a minimal width comp (I think it's 4 px). Adjust the anchor point of the precomp's layer in the main comp to the edge, turn the layer's quality switch to draft, and scale it way up horizontally (but not vertically). If you want to animate, you can link the anchor point of the precomp's source to the precomp layer's position.
Make sure when you render that you modify the render settings! "Best Settings" will override your draft-quality scaling. Choose "Current Setttings" for quality in the render settings.
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