Windows build fix for tools/RayTracer.h and changes from Houdini trunk

Note that the changes to GEO_PrimVDB/GU_PrimVDB are mainly for compiling in H12.1 since we no longer compile the primitive in H12.5. So, changes in those classes that can only be compiled for H12.5/H13.0 have not been merged here. Just thought I'd mention this if you compare these headers with the ones in the HDK.