Integrate AMD FidelityFX Single Pass Downsampler in the Vulkan renderer

[Calinou]

See also #1519 and #2809.

Describe the project you are working on

The Godot editor 🙂

Describe the problem or limitation you are having in your project

The mipmap generation was sped up in master already, but there are opportunities to make it even faster.

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

Integrate AMD FidelityFX Single Pass Downsampler to further speed up real-time mipmap generation.
This should improve performance when using ReflectionProbes and sky shaders.

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

I'm not sure how to implement it (or even how it differs from our current implementation), unfortunately.

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

No, as this downsampling is done using low-level Vulkan operations which cannot be replicated in an user-provided shader.

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

This is core rendering functionality.