Blender Git Loki

DerivedRender cleanup and improvements

- Use special flags in DagNode for tags and scheduled
instead of trying to use color for this. This way
it's possible to have both tag and scheduler working
at the same time.

- Simplified DAG threaded traversal code, moved comments
to depsgaph instead of documenting what functions does
when using them.

- Switch from recursive tag flush to iterative one.

- Set scenes shall be handled properly in convertblender.

- Removed derivedRender construction from handle object
update function. It doesn't make much sense to have it
in both there and convertblender.

- Creating new task pool now ensures malloc is switched
to thread-safe one.

- Create derivedRender in threads using the same way as
it's done for object update. In really quick tests
gave around 2x speedup of database_init_objects when
having 130 suzannes with subsurf level of 3.