DesMuMe_Libretro Portrait Mode (Top/Bottom)

I finally managed to get a rotatable stand for my monitors (surprisingly hard to get around here O.o) and i was wondering if there are settings to take advantage of this.

The Retroarch menu allows me to tilt the screen 90°, but it squishes the picture, basically keeping the aspect ratio for a standard vertical display. The aim here is to have the regular Top / Bottom screen setup, only in “Tate” (Portrait) mode, hopefully without having to fiddle with my driver settings, since its a temporary setting just for certain games.

Thanks in advance !