Improve shadows for models that don't have normals

The shadowing system relies on the presence of normals for best results. Photogrammetry tilesets typically don't have normals and as a result have messy shadows. It may be possible to use different shadow bias settings for models without normals.

The issue names the shadowing system but no files, tests, or entry points. Locate the shadowing implementation and investigate how models without normals are handled; done means photogrammetry tilesets produce clean shadows without normals, with suitable regression coverage if the project provides it.

Written by the indexing model from the issue text.