IBL artifacts on model with varying roughness

[jjhembd]

What happened?

As reported by @ggetz in #11988, the Barn Lamp sample model shows some artifacts when rendered with an environment map:

This model has a lot of variation in the roughness map. Here is the roughness as displayed in the glTF Sample Viewer:

The artifact could be related to LOD sampling of the environment map. Rougher materials sample a lower-resolution version of the environment map.

Reproduction steps

1. Load the glTF PBR Extensions Sandcastle
2. View the "Barn Lamp" model with "Environment map lighting"

Sandcastle example

https://sandcastle.cesium.com/?src=glTF%20PBR%20Extensions.html

Environment

Browser: Chrome
CesiumJS Version: 1.119
Operating System: Windows 11