openvdb::tools::meshToSignedDistanceField does not support anisotrop voxel sizes

I tried using meshToSignedDistanceField with a world to index transform that maps to
an anisotrop voxel size (e.g. 0.5, 0.25, 0.25). When reading the resulting distance field,
I noticed that the distances are wrong, because only the voxel size in X dimension is taken into account:

`const ValueType voxelSize = ValueType(transform.voxelSize()[0]);
`
Could you maybe support anisotropic voxel sizes in a future version?

Start at the meshToSignedDistanceField entry point and inspect the reported voxelSize use, especially the conversion that reads only the X dimension. Confirm the distance field remains correct when the world-to-index transform has different voxel sizes on each axis, then verify the relevant existing tests or add coverage for the anisotropic example.

Written by the indexing model from the issue text.