Implement FSR 1.0 Lite in the Vulkan renderer

[Calinou]

Describe the project you are working on

The Godot editor 🙂

Describe the problem or limitation you are having in your project

Godot's Vulkan Clustered backend now offers support for AMD FSR thanks to godotengine/godot#51679. There are two caveats though:

• This FSR implementation is only available when using the Vulkan Clustered backend, not the Vulkan Mobile backend.
• The "full" FSR implementation is too heavy to be usable on mobile and integrated graphics.

Describe the feature / enhancement and how it helps to overcome the problem or limitation

Add a "FSR Lite" scaling option as demonstrated here: https://atyuwen.github.io/posts/optimizing-fsr/

This FSR Lite implementation should ideally be made available both in the Vulkan Clustered and Vulkan Mobile backends. It should be exposed as a separate option from "full" FSR scaling. Also, when the user selects "full" FSR scaling on the Vulkan Mobile backend, it should fall back to FSR Lite.

FSR Lite looks a bit worse than vanilla FSR, but it performs much better on mobile GPUs and integrated graphics. It's fast enough to be usable on mid-range/high-end mobile.

Describe how your proposal will work, with code, pseudo-code, mock-ups, and/or diagrams

See the commits that added FSR Lite to the original FSR sample: https://github.com/atyuwen/FidelityFX-FSR/commits/master

If this enhancement will not be used often, can it be worked around with a few lines of script?

No, as FSR needs to be implemented in the core renderer for an optimal implementation.

Is there a reason why this should be core and not an add-on in the asset library?

FSR can't be optimally implemented with an add-on.