OpenVDB:
 - Improvements:
   - Significantly improved the performance of all LeafNode ValueIterators,
     up to 5x on some platforms and up to 10x when delay loading is enabled.
     Construction of a ValueIterator from a leaf node now requests the leaf
     buffers ahead of iteration to avoid potentially expensive API calls.
   - Added OPENVDB_ASSUME macros to mimic builtin assume and C++23 assume
     attributes.
