Blender Git Loki

Cycles: Wrap spatial split storage into own structure

This has following advantages:

- Localizes all the run-time storage into a single structure,
which could easily be extended further.

- Storage could be created per-thread, so once builder is
threaded we wouldn't have any conflicts between threads.

- Global nature of the storage avoids memory re-allocation
on the runtime, keeping builder as fast as possible.

Currently it's just API changes, which don't affect user at all.