Blender Git Loki

Fixes T77882: artifacts rendering OpenVDB volumes with multiple grids in Cycles

The previous algorithm was not using all of the requested grids to build a mesh
around the volume due to limitations regarding the use of a dense buffer to
gather information about the volume's topology. This resulted in artefacts during
rendering.

The mesh generation is now done by merging all of the input grids and using the
resulting grid's topology to create the mesh. The generation of the mesh
is still done in index space as before, and the vertices are converted to object
space by using the merged topology grid indexToWorld transform.

To be able to merge the grids together we have to make sure that their transformation
matrices and their index spaces match, thus, if they do not match we simply resample
the grids. This behaviour should tackle one other limitation of the current algorithm,
which is that only one transformation matrix was used to generate the final mesh.

If we do not have an OpenVDB grid for the requested volume data, we generate
a temporary OpenVDB grid for it.

Differential Revision: https://developer.blender.org/D8401