Introduce ExposureShader

[WestLangley]

This is an exposure-only pass. Useful for rendering HDR output to an archive EXR in working color space, for example.

As suggested in #26130 (comment).

[mrdoob]

Hmm, actually... Wouldn't it make sense to do this also in the OutputPass?

[WestLangley]

@mrdoob Can you please explain what you are referring to?

[mrdoob]

Oh, actually... Never mind, I misunderstood.

[mrdoob]

No wait...

Wouldn't it be better to hide this for the developer in OutputPass?

Ideally the developer wouldn't have to have two different code paths (One for HDR and another for non-HDR).
They would just use OutputPass. And OutputPass would do toneMapping only in non-HDR displays.

[WestLangley]

This is separate from OutputPass, which is device-specific.

This is an exposure-only pass. Useful for rendering HDR output to an archive EXR in working color space, for example.

[mrdoob]

Ah okay... Makes sense.

I think I didn't understand the exact use case for this PR by reading the conversation #26130.

I'll update your PR description for clarity in case we have to look at this PR again in the future.