Blender Git Loki

Cycles: load OpenVDB file earlier in Blender export

In an upcoming bugfix we'll use OpenVDB data structures directly to build mesh
for sparse OpenVDB volumes, loading them OpenVDB grids earlier and removing any
references to Blender data structures makes that easier.

This also makes changes to Blender volumes to support this, so Cycles can take
ownership of a grid without Blender having to keep its own reference to it.
This should also be useful in a future Python API.

Ref D8401