Introduce Scene.backgroundIntensity

[WestLangley]

Since background textures can be HDR, we need to be able scale the background texture's brightness independently from the scene itself.

Just as we have material.envMapIntensity, this PR adds scene.backgroundIntensity.

Perhaps there is a better name for this, but I do not see how this property can be avoided.

Comments welcome.

[donmccurdy]

To my understanding, it's common for HDRIs to be "normalized", with units relative to some chosen baseline. In that case I would expect to see correct exposure by default. The main case I'd imagine where the background intensity must be adjusted is when using an HDR background alongside physically based lights or modified renderer.toneMappingExposure settings, to counteract the exposure and bring the (non-physical) values of the HDRI back into the original range.

The proposed naming sounds good to me. I was also thinking of .backgroundExposure, but not sure which word is better as the background is neither a light source nor a camera... Maybe just consistency w/ envMapIntensity is best.

[WestLangley]

Thanks! It looks like there is agreement. :-)

[Mugen87]

Merging this to complete HDR support for Scene.background.

[Mugen87]

Since we are adding new properties to Scene, the suggested design in #17420 makes more and more sense. At some point, we should consider to refactor Scene.background so it works with ColorBackground, TextureBackground and CubeTextureBackground (the last name potentially needs an update since you can assign equirect and cubeuv textures, too).