[Feature Request] Fake Fullscreen option for games with broken or non-existent fullscreen

Some games such as Knight of the Old Republic have known issues with fullscreen mode in both Windows, upstream WINE and Proton - for the aforementioned game, the only way to reliably run the game is the set it to run in windowed mode. Without mods, 1600x1200 is the highest resolution.

On a 1440p screen that's pretty ugly looking, for a 4K screen that's awfully small, I'd hate to see it on 5K or 8K...

An option to allow windowed games to be upscaled to fullscreen (by Proton/Steam) would alleviate this and could as an added bonus be used for upscaling games with low resolutions to higher screen resolutions with nice filters like lanczos or spline36 (possibly with a low-impact bilinear option too)

GeDoSaTo is possibly a good point of reference (though it typically is used for supersampling - that wouldn't go astray either, Linux support for VSR on AMD is non existent, not sure for DSR on nVidia - but that might be out of the scope of this request whilst tangibly related)

The issue body is the only implementation guidance and does not name files, tests, or entry points. Start by determining where Proton handles game window sizing and presentation, then define the fullscreen-upscaling scope and verify the requested behavior with a game that lacks reliable fullscreen support.