Blender Git Loki

Update metaballs in a single thread

Mballs evaluation uses BKE_scene_base_iter_next which calls
duplilist for all objects in the scene. This leads to conflict
accessing and writing same data from multiple threads.

Ideally Mballs shouldn't do such an iteration and use DAG
queries instead. For the time being we've got new DAG
let's keep it simple and update mballs in a ingle thread.