EXRLoader: Copy Y to RGB channels and return RGBAFormat

[mrdoob]

Related issue: #31429 #31482

Description

@sciecode Also made with Claude! 🤓 Looks good?

/cc @Mugen87 @drone1

[sciecode]

Close, but no cigar.

It's not handling HalfFloatType alpha, that needs to be 0x3C00.

And trying to expand during decoding isn't great, in case we need to change / implement extra features in the future. Better to do it once decoding is done.

[sciecode]

Mugen87 also suggested that we made the extension optional (but defaults to true), which I think it's probably a good idea.

[drone1]

@mrdoob I realize you're still working, but if you think it'd be helpful for me to pitch in with a test in examples to keep an eye on this issue going forward, lemme know and I can do that.

It looks like your changes have at least fixed the issue for the specific .exr roughness map I referenced.

I could potentially do some more testing with other textures and submit a PR. If it makes more sense for you or someone else to do it, no worries ofc.

EDIT: I kinda just did it to make the screenshot in the end haha. Let me know if I can help :]

Looks like the asset is OK to include with three.js (pasted the license into Claude), and I credited the artist.

[mrdoob]

I don't think we need to update the example.

However, just added support for exr to the editor (01d3c94) to make testing easier: