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Volumes: use bounding box diagonal to compute adaptive voxel size

Using the diagonal has the benefit, that the adaptive voxel size changes
when the bounding box changes its shape in any way. Having a changing
voxel size looks bad when rendering an animation, therefore one should
usually use a fixed voxel size when rendering an animated volume.
This becomes apparent earlier, when the adaptive voxel size changes
when the bounding box changes in any way.
Otherwise this can easily go unnoticed when rendering only a few frames
of an animated volume.