CatFrame Universal Model Rendering Extension System

Important

This document has been merged into the "Rendering Extension System" section of MODEL_SYSTEM.md. That document now contains complete model system and rendering system documentation.

Applicable to all rendering of blocks (in the world) and items (GUI / held) under the CatFrame JSON model pipeline. Design goal: allow mods to insert custom tinting, shadows, self-illumination, face culling, and other rendering effects through a unified registration point, without needing to mixin the rendering pipeline.