The smoke obstacle detection was using a maximum distance for BVH
checks to find mesh elements that define boundary cells in the grid.
This BVH test was using an arbitrary value of 0.6 cell units. It should
be `sqrt(3)*0.5` to account for the maximum possible distance of mesh
elements inside a cell. Otherwise some cells that should form the
boundary are not detected as such (no closest mesh element found inside
the radius), so you get gaps in the smoke obstacle.
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