EEVEE-Next can only store data for a single (global) closure at a time,
which, when combined with ShaderToRGB nodes, requires extra care in the
order that closures are evaluated.
For example:
Here, after `ntree_shader_shader_to_rgba_branch` there will be 2 Diffuse
nodes (the original and a copy for the ShaderToRGB branch).
However, the generated code order will be something like this:
```
Diffuse (original)
Diffuse (copy)
ShaderToRGB // This resets closures
Mix
```
So while the original node is technically "evaluated", the closure data
is reset after ShaderToRGB.
This patch updates the code generation to ensure closure evaluation is
ordered taking ShaderToRGB branches into account, so the generated code
looks like this:
```
Diffuse (copy)
ShaderToRGB // This resets closures
Diffuse (original)
Mix
```
This also fixes ShaderToRGB support for AOVs, removes unused code, and
fixes several bugs that I've found along the way that were harmless for
EEVEE but broke EEVEE Next.
Pull Request: https://projects.blender.org/blender/blender/pulls/117767
7e8fd2cc2c