Async. compilation via ubershader is currently available in the scene and particles shaders only.
Bonus:
- Use `#if defined()` syntax for not true conditionals, so they don't unnecessarily take a bit in the version flagset.
- Remove unused `ENABLE_CLIP_ALPHA` from scene shader.
- Remove unused `PARTICLES_COPY` from the particles shader.
- Remove unused uniform related code.
- Shader language/compiler: use ordered hash maps for deterministic code generation (needed for caching).
Also include public domain assets in `COPYRIGHT.txt` with Unlicence text or
dual-licensing scheme.
And document commit hashes for most thirdparty code in `thirdparty/README.md`
for clarity, and in case there's no tag matching the included version numbers.
(cherry picked from commit c6802a65c6)
