[Feature] Support asymmetric voxel sizes in meshToVolume()

[diiigle]

Hi,

coming from the ASWF presentation you gave the other day. Thanks for putting that together, that was really insightful and gave a nice overview of what is going on. I actually have a feature idea lying around for quite a while:

In 3D printing (research) you quite often end up with asymmetric voxels due to the way the hardware is build (e..g 150x300x421 dpi printer resolution). This is generally not a problem for openvdb grids, but the voxelization step in MeshToVolume.h can only work with symmetric voxels.

Specifically this line

openvdb/openvdb/openvdb/tools/MeshToVolume.h

Line 3162 in af1e425

const ValueType voxelSize = ValueType(transform.voxelSize()[0]);

in meshToVolume() takes only the x component which is then used from there on.

I believe adding this support would involve passing the full 3 component voxelsize to all functions & operators and modifying the 7 occurrences of lengthSqr (where indexSpace == distanceSpace is assumed) to compute the correct distance values for the (signed) levelset / distance field.

What do you think?