Base closure pointer in layer is incorrect when using batched execute

### Problem

Base closure pointer in osl layer points to its parent layer closure when using batched execute while it should actually point to a diffuse closure.

```
void process_bsdf_closure(const ClosureColor* closure) {
const ClosureComponent* comp = closure->as_comp();
const MxLayerParams* srcparams = comp->as();
assert(srcparams->base != closure);
```

[minimal_test_case.zip](https://github.com/AcademySoftwareFoundation/OpenShadingLanguage/files/15049651/minimal_test_case.zip)

### Steps to Reproduce

Download the attached minimal sample and issue the following commands:

g++ -DUSE_BATCHED -mavx2 -mfma -mtune=skylake -mf16c -g test2.cpp -loslcomp -loslexec -loslnoise -loslquery -lOpenImageIO -lOpenImageIO_Util
./a.out
parsing mx layer closure
parsing generalized schlick closure
Error: base closure in mx layer points back to currently parsed closure

g++ -mavx2 -mfma -mtune=skylake -mf16c -g test2.cpp -loslcomp -loslexec -loslnoise -loslquery -lOpenImageIO -lOpenImageIO_Util
./a.out
parsing mx layer closure
parsing generalized schlick closure
parsing diffuse closure

Here's .osl shader codes in the .zip:

```
shader layer_test(
color base_color = color(0.8, 0.8, 0.8),
color specular_tint = color(1.0),
float roughness = 0.5,
normal Tangent = normalize(dPdu),
output closure color BSDF = 0)
{
BSDF = base_color * diffuse(N);
color F0 = specular_tint;
color F90 = color(1.0);
BSDF = layer(generalized_schlick_bsdf(N, Tangent, color(1.0), color(0.0), roughness, roughness, F0, F90, 0.0, "ggx"),BSDF);
}
```

```
surface output_surface(closure color Surface = 0)
{
Ci = Surface;
}
```

### Versions

* OSL branch/version: v1.13.8.0
* OS: linux 64-bit 5.10.0-18-amd64
* C++ compiler: g++ 10.2.1
* LLVM version: 11.0.1-2
* OIIO version: v2.5.4.0

Beginnen Sie mit dem angehängten minimalen Testfall, insbesondere mit test2.cpp, und reproduzieren Sie den Fehler mit aktiviertem USE_BATCHED und ohne. Verfolgen Sie process_bsdf_closure, MxLayerParams und die Behandlung von Layer-Closures in osl; als erledigt gilt die Aufgabe, wenn der gebatchte Lauf die diffuse closure als Basis meldet und die bestehende Assertion nicht mehr fehlschlägt, während die Ausgabe ohne Batching korrekt bleibt.

Vom Indexierungsmodell aus dem Issue-Text verfasst.