Blender Git Loki

Improved thread data handling for the OpenVDB Cycles node.

OpenVDB grid accessors, samplers, and ray intersectors must be stored per thread.
Previously this data was stored in a per-thread map in each grid. This causes problems
because the pthread_id keys can become invalid, and it also creates a lot of unused
accessors.

The patch switches thread data storage around, so that thread-local data is now passed
along KernelGlobals for every work task as needed (tasks only run in one thread).
This system is much more reliable and similar to other cases of thread-local data,
such as OSL shaders.

The patch also removes the virtual base class for "volumes", which might give a minor
performance improvement. If a generic volume type for Cycles is needed it can be
abstracted from scratch.

Removed the unused thread_id list in Cycles CPU tasks, which were only needed
for the now deprecated OpenVDB grid accessor lookup.