Sample screenshots have a skybox for models that don't need one

The sample screenshots should only use a skybox for the models that have features that require a skybox (Highly metallic, low alpha, etc...)

Start by locating the sample screenshot generation entry point and the model definitions that control skybox use. Compare the screenshots for models with and without skybox-dependent features, then verify that only the models requiring a skybox use one.

Written by the indexing model from the issue text.