Add screenshot testing for subtle lighting changes

[jjhembd]

Feature

Changes to the rendering pipeline can result in subtle changes to lighting, especially in models using physically-based rendering. See #10346, where manual testing revealed subtle rendering differences resulting from a refactor of the pipeline.

Catching these changes can be difficult because we need to manually inspect and compare the rendering of a wide range of models. This process would be faster and more robust if we automate it via end-to-end screenshot testing.

Some ideas for these tests:

1. Render test models without any terrain or imagery in the background. This will make the tests less sensitive to network delays.
2. Use a very low tolerance for screenshot comparisons. Lighting effects can be subtle.