Scene: Add background blur with PMREM.

[Mugen87]

Related issue: #23712

Description

This is the alternative implementation of #23712 but via PMREMs.

[WestLangley]

I support your PR in principle, but not how it has been shoehorned into the existing shaders.

I have been trying to clean up the shaders, and this is not helping.

I am hopeful that if this is merged, we can work together on some shader refactoring.

[Mugen87]

Definitely. IMO when adding new features, there is no need to make the implementation perfect right from the beginning. As long as the basic approach is right, we can use refactoring to fine-tune things at a later point.

[Mugen87]

Let's merge so we can easier move on with the suggested refactoring.

[Mugen87]

Updated builds: 4e93f9b

[Mugen87]

About the refactoring: I guess the goal is that the background material does not depend on ShaderLib.cube anymore. Right now ShaderLib.background is only used for simple textured backgrounds. Are you okay with extending it so it also handles env maps?

[WestLangley]

ShaderLib.background is only used for simple textured backgrounds. Are you okay with extending it so it also handles env maps?

I think that would be OK. I am assuming it would be used exclusively by scene.background.

so it also handles env maps...

Nit: I would prefer to say it this way: "so it also handles texture cubes and equirectangular textures".