Merged changes from SESI for compatibility with Houdini 13.
Added a simple multithreaded VolumeRender to RayTracer.h.
Various fixes
Merge remote-tracking branch 'upstream/master'

Conflicts:
	openvdb/Platform.h
	openvdb_houdini/houdini/GU_PrimVDB.cc
	openvdb_houdini/houdini/GeometryUtil.cc
	openvdb_houdini/houdini/GeometryUtil.h
	openvdb_houdini/houdini/SOP_OpenVDB_Analysis.cc
	openvdb_houdini/houdini/SOP_OpenVDB_Convert.cc
	openvdb_houdini/houdini/SOP_OpenVDB_Filter.cc
	openvdb_houdini/houdini/SOP_OpenVDB_Filter_Level_Set.cc
	openvdb_houdini/houdini/SOP_OpenVDB_Ray.cc
	openvdb_houdini/houdini/SOP_OpenVDB_To_Spheres.cc
	openvdb_houdini/houdini/SOP_OpenVDB_Vector_Split.cc