Could we get a new setting inside pixel size labeled "custom" where we can input the exact resolution we want shaderglass to downsample the game/video/app to?
For instance, some games, like CrossCode, use a weird arbitrary resolution like 568x320 that none of the pixel size presets in shaderglass lines up properly with no matter what i do. However, if i could go into pixel size and pick 'custom' and enter 568 x 320 it would downsample to that and it should work with games like that.
and as a bonus, perhaps an X and Y offset input box for those pesky games whose entire pixel grid is centered funny on the output and ALL the pixels are like, 'halfway off' the grid.. shovel knight kinda is like that.
in general, it can be like shooting in the dark when it comes to figuring out the resolution purely by 1x, 2x, 4x etc.. especially since there's no indication on what resolution any of those are actually downsampling to.
There's been a discussion on GitHub about this, the crux of the matter is that if you don't have integer pixel size it might be difficult for ShaderGlass to match the source scaling logic exactly (as you say it might require having additional offset settings etc.); for games in weird resolutions running them windowed at x1 pixel and then Input -> Window mode in ShaderGlass + fullscreen through ShaderGlass is the preferred route atm.