Our deformer system really needs some work. First, there was a crash
with shape keys because BL_ModifierDeformer derives from
BL_ShapeDeformer, which means we try to execute shape keys even if we do
not have them. Also, for some reason BL_ModifierDeformer::Update() does
not work if called from the threaded loop, so it is skipped for now. In
other words, skinned updates on meshes with modifiers are currently not
run in parallel.
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