save memory during render and cache render results.
Implementation notes:
In the render engine API it's now possible to get the render result for
one render layer only, and retrieve the expected tile size in case save
buffers is used. This is needed because EXR expects tiles with particular
size and coordinates.
The EXR temporary files are now also separated per layer, since Cycles
can't give the full render result for all render layers, and EXR doesn't
support writing parts of tiles.
In Cycles internally the handling of render buffers and multi GPU
rendering in particular changed quite a bit, and could use a bit more
refactoring to make things more consistent and simple.
* Passes renamed to samples
* Camera lens radius renamed to aperature size/blades/rotation
* Glass and fresnel nodes input is now index of refraction
* Glossy and velvet fresnel socket removed
* Mix/add closure node renamed to mix/add shader node
* Blend weight node added for shader mixing weights
There is some version patching code for reading existing files, but it's not
perfect, so shaders may work a bit different.
* Fix missing update when editing objects with emission materials.
* Fix preview pass rendering set to 1 not showing full resolution.
* Fix CUDA runtime compiling failing due to missing cache directory.
* Use settings from first render layer for visibility and material override.
And a bunch of incomplete and still disabled code mostly related to closure
sampling.
* 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.
