* If a Background node is set to a black color or zero strength,
it now gets removed from the shader graph.
* In case the graph is empty (no background node), the kernel will skip
evaluating it and save some rendertime. This can help quite a bit in scenes,
where the majority of the image consists of a black background.
Example: http://www.pasteall.org/pic/show.php?id=82650
In this case the render is ~16% faster.
Differential Revision: https://developer.blender.org/D972
cd72396797