Vulkan: LightmapGI bicubic sampling not implemented (unlike BakedLightmap in 3.x)

[Calinou]

See also #49968, #50568 and #52199.

Godot version

4.0.dev (659178e)

System information

Fedora 34, GeForce GTX 1080 (NVIDIA 465.31)

Issue description

In the master branch, baked lightmaps can only use bilinear sampling instead of bicubic sampling. This results in lower visual quality, especially when using the Static (formerly All) bake mode:

With bicubic sampling, aliasing in the lightmap becomes much less noticeable. Once it's reimplemented, it should also be enabled by default like in 3.x.

Steps to reproduce

Look for a lightmap bicubic sampling project setting (as is present in 3.x).

Minimal reproduction project

test_lightmap_normalmap.zip

[Calinou]

I've started to implement this: https://github.com/Calinou/godot/tree/lightmapgi-add-bicubic-lightmap-sampling

I feel like it doesn't look as smooth as it should be, but it's still a marked improvement: https://imgsli.com/Njc2NzQ