It seems that a few cores are using the wrong resolution to calculate the aspect ratio. For example, Genesis Plus assumes that 256x240 is the 1x1 scale resolution for all games, but most games use 320x240 instead. This problem don’t occur with Picodrive, which uses 320x240.
The 1x1 resolution scale for Mednafen PCE is defined to 341x232, which looks wrong, since most PCE games use 256x224 instead.
SNES9x Next is using 256x239, but bSNES is using a more suitable 256x240 (and it looks better).
Is it possible for Retroarch to calculate the exact resolution of each game and display it as it is, without any scaling?
Mednafen PSX is using the most common PSX resolution (320x240), but running at this exactly resolution is producing a lot of scaling artifacts with an unfiltered image. With games running at native 320x240, the only way to have a picture without scaling problems is to turn off the integer scale option and create a custom ratio of 350x240. Is that a bug?
EDIT: I solved Mednafen PSX problem by turning on Crop Overscan.