The default lighting might be too dark

There are several forum threads that talk about models in general appearing "too dark".

_Note:_ It might be that some of these issues are caused by https://github.com/CesiumGS/cesium/issues/12310 under the hood: When the environment map is rotated, then it could mean that, very roughly speaking, the model only receives the light from "the floor" where it should receive the light from "the sky", which might cause it to look too dark.

Regardless of that, we might have to investigate whether there are certain default values used in the image based lighting computations that have to be reviewed. I know, lighting is difficult (light intensity, lumens, lux, candela, exposure, energy conservation...), and carving that into shader code is another level. But maybe there is something that could guide us to some sort of "objective truth"...

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Just a few examples for forum threads:

- https://community.cesium.com/t/dark-models-depending-on-time-of-day/12097 - This may have partially been caused by the time-of-day, but the model still appeared pretty dark, and other users chimed in there reporting similar problems
- https://community.cesium.com/t/3d-tile-point-cloud-looks-bad-in-new-version/40452 - This may also _partially_ be caused by the time-of-day, but even in the brightest daylight, the model appeared darker than in previous versions
- https://community.cesium.com/t/change-sun-light-to-white-light/39105 Where the lighting in general seemed to be not "white/bright" enough, with several workarounds being suggested
- https://community.cesium.com/t/color-differences-after-uploading-a-model-to-cesium/39541 Where models have been described as ~"a bit dark and not very vibrant"
- https://community.cesium.com/t/glb-model-too-dark/40656 Where models have been decribed as "too dark", also with some possible workarounds

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Start by reproducing the reported dark-model cases from the linked forum threads and comparing behavior across time of day. Investigate the image-based lighting defaults and the environment-map rotation concern in issue 12310. Done means determining whether the defaults or rotation explain the darkness and identifying a defensible direction for reviewing them.

Written by the indexing model from the issue text.