Add option for clay render

Would essentially make all textures gray. With the exception to alpha textures.

See attached comparison for what this might look like in use.

No files, tests, or entry points are named. Start by locating the render-option handling and texture sampling code, then trace how alpha textures are distinguished; done means an enabled clay-render option makes non-alpha textures gray while preserving alpha textures, with coverage for the new behavior.

Written by the indexing model from the issue text.