Provide implementation guidance on unlit material composition

The `KHR_materials_unlit` extension replaces the PBR color definition with only the base color value. At the same time, the list of use cases includes photogrammetry, which may imply that a mesh is not only unlit but also bypasses all tone-mapping and/or exposure adjustments and thus should be rendered as-is.

If an unlit material is used in a scene that also contains regular PBR materials, this issue may lead to various problems:
- if tone-mapping and/or exposure steps are applied, then photogrammetry content may look under- or overexposed;
- if tone-mapping and/or exposure steps are skipped, then accurate scene composition may be prohibitively expensive or even impossible if PBR materials use transmission.

/cc @emackey

No files, tests, or implementation entry points are named. Start by reviewing the KHR_materials_unlit extension and the listed photogrammetry and PBR composition concerns, then define the implementation guidance and rendering behavior that would resolve the open design questions.

Written by the indexing model from the issue text.