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Cycles: Ignore zero size instances in BVH

In certain types of animation it's possible to have some objects
scaling to zero. In this case we can save render times by avoid
traversing such instances.

Better to do ti ahead of a time, so traversal stays simple.

Reviewers: lukasstockner97, dingto, brecht

Reviewed By: brecht

Differential Revision: https://developer.blender.org/D2048