Display density metadata in the .vdb file format

I have a suggestion to improve viewport previews for VDB files in houdini.

Currently if we use the .vdb file format to write out volumes, the display density set by the primitive node before writing .vdb isn't respected upon reading a vdb which makes viewport previews and reading .vdb files not nice. It would be great if some metadata could be added to improve viewport previews when reading VDB files if it isn't already part of the .vdb standard. This would aid software like Side FX's Houdini in preserving display of volumes between caching steps.

The issue proposes adding display-density metadata to the .vdb format so Houdini can preserve volume viewport previews across caching steps. Start by determining whether this metadata is already part of the VDB standard and where VDB metadata is defined and read or written. Done means agreeing on the metadata representation and implementing compatible preservation for the described workflow.

Written by the indexing model from the issue text.