Include launch option to brute-force total, bordered fullscreen

Feature Request

I confirm:

• that I haven't found another request for this feature.
• that I have checked whether there are updates for my system available that
  contain this feature already.

Description

FSR is an useful feature that allows players to get better performance in games out of weaker GPUs. I've used it on The Outer Worlds to great effect with my RX 570, getting high performance on Very High settings despite my 4 GBs of VRAM and added DXVK overhead. However, the main issue with FSR is that it only works on games that run in complete fullscreen mode... and the majority of games nowadays have abandoned that option in favour of only running on Borderless Fullscreen, meaning that FSR will never work with them.

Because of this, I think it could be worthwhile to invest in an option that makes Proton brute-force a Borderless Fullscreen game to run on complete Bordered Fullscreen instead, allowing FSR to be used in it. It's likely not as easy to implement as I hope it is, but FSR is such an essential feature for lower-end gamers (especially given how insane the GPU prices are at the moment) that I really wish a feature like this could be implemented.

Justification

Proton is, first and foremost, a tool designed for gaming. Wine is not. Therefore, it's highly unlikely that such a feature would be accepted into vanilla Wine as it does not fit their goals, but it could potentially be very helpful for Proton as it'd make Steam gaming even better for most gamers unable to afford top-of-the-line GPUs.

Risks

Brute-forcing an app to run on Bordered Fullscreen may not be as simple as it sounds, I will admit. It might also potentially not help with making FSR functional at all. However, it ultimately could be a good first step in improving FSR compatibility.

The request names no files, tests, or entry points. Start by locating Proton's fullscreen handling and FSR-related integration, then determine whether an option can force bordered fullscreen; done would require a working implementation with verified FSR behavior.