Blender Git Loki

Fix 'BLI_task_parallel_mempool' keeping 'user_chunk' unchanged

When `BLI_task_parallel_mempool` does not use threading, the
`userdata_chunk` is allocated locally simulating a TLS.

However `func_reduce` is not called so the original chunk is ignored.

`task_parallel_iterator_no_threads` is another function that doesn't call
`func_reduce`. It also ignores `userdata_chunk_local` in the main iterator.

The solution in these cases is not to create a `userdata_chunk_local`.

This fixes T90131

Differential Revision: https://developer.blender.org/D12067