Invert X-Axis for the Screen

[ymn 0]

I'm trying to do what was attempted here

but the methods on there are don't really work.

If I try to get the pixels from the screen, then every other iteration, I'll get my already mirrored pixels back, and I'll re-mirror them, making the screen wildly switch between normal and mirrored (you can see this is in one of the scripts on the linked thread). Is there a reasonable way to somehow get the unaffected-by-my-program pixel information? (I'm leaning towards no)

Looping over pixels is slow anyway. Is there some DLL out there that will do it for me? I know there's one for rotation, but I haven't seen one for flipping.