Armatures used to check if any of their meshes were culled to see if they needed
to be updated. However, this meant armatures with no meshes would never update,
since non-mesh objects are always considered culled. Instead, if a non-culled
child was not found, we now check to see if the armature contained only
non-mesh objects. If this is the case, always update the armature. If this
becomes a problem, we can look into being able to cull non-mesh objects.
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