Blender Git Loki

Add a new parallel looper for MemPool items to BLI_task.

It merely uses the new thread-safe iterators system of mempool, quite
straight forward.

Note that to avoid possible confusion with two void pointers as
parameters of the callback, a dummy opaque struct pointer is used
instead for the second parameter (pointer generated by iteration over
mempool), callback functions must explicitely convert it to expected
real type.

Also added a basic gtest for this new feature.