[VFX] Fix SpawnIndex attribute when using instancing

We are storing the wrong value for SpawnIndex attribute during Init compute, when processing several instances at the same time.
We are currently using the batch thread index, instead of per instance thread index.
The fix is very simple, just replacing one with the other when storing the value.

Packages/com.unity.visualeffectgraph/Shaders/VFXInit.template

@@ -138,7 +138,7 @@ void CSMain(uint3 groupId          : SV_GroupID,
         attributes.seed = WangHash(particleIndex ^ systemSeed);
 #endif
 #if VFX_USE_SPAWNINDEX_CURRENT
-        attributes.spawnIndex = id;
+        attributes.spawnIndex = index;
 #endif
 
 #if VFX_USE_SPAWNER_FROM_GPU && VFX_USE_SPAWNCOUNT_SOURCE