Flip image horizontally in some games
As you may know, some lightgun games appear horizontally flipped in MAME 2003 as they originally used a mirror to display the image. I've been looking for a solution to this, and finally I found a shader which fix it perfectly: https://github.com/libretro/glsl-shaders/blob/master/misc/flip-horizontal.glsl
Unfortunately, it's not included in Retropie, so you must download it and put it in /opt/retropie/configs/all/retroarch/shaders. Once there, run the game and, in Retroarch menu, go to Shaders, select it, 'Apply Changes' and then 'Save game preset'.
@svera i think mame2003plus has corrected a lot of these flipped games.
@dankcushions I did try with MAME2003plus and Operation thunderbolt and the issue persisted, don't know if other games are fixed.