The OpenVDB data structure can store voxel data in leaf nodes or tiles
when all the nodes in a given region have a constant value. However,
Cycles is using the leaf nodes to generate the acceleration structure
for computing volume intersections which did not include constant tiles.
To fix this, we simply voxelize all the active tiles prior to generating
the volume bounding mesh. As we are using a MaskGrid, this will not
allocate actual voxel buffers for each leaf, so the memory usage will be
kept low.
Differential Revision: https://developer.blender.org/D9557
Reviewed by: brecht, JacquesLucke
13ec512f4b