Blender Git Loki

Cycles: Resolve ridiculous amount of memory used by spatial split builder

This was only visible on systems with lots of threads and root of the issue
was that we've been pre-allocating too much memory for all the threads.

Now we only pre-allocate data for the main thread and rest of the threads
does allocation on-demand.

This brings down memory usage from 36Gig to 6.9Gig when building spatial
split for the Bunny.blend file on our Intel beast.

Originally regression was happened by the threaded spacial split builder
commit.