eaa87101cd made evaluated metaballs become meshes
for render engines. This exposed the normals calculated by the metaball
tessellation directly, whereas before they were recalculated later.
There are two fixes: not using the metaball tessellation normals at
all and calculating them with the standard Mesh code path instead,
or switching a define in the metaball code to use a more mesh-like
vertex normal accumulation. Since the results are very similar
(see attached renders), use the second solution, which should
be less expensive.
Hans Goudey