* 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
* Add alpha pass output, to use set Transparent option in Film panel.
* Add Holdout closure (OSL terminology), this is like the Sky option in the
internal renderer, objects with this closure show the background / zero
alpha.
* Add option to use Gaussian instead of Box pixel filter in the UI.
* Remove camera response curves for now, they don't really belong here in
the pipeline, should be moved to compositor.
* Output full float values for rendering now, previously was only byte precision.
* Add a patch from Thomas to get a preview passes option, but still disabled
because it isn't quite working right yet.
* CUDA: don't compile shader graph evaluation inline.
* Convert tabs to spaces in python files.