* 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.
* add some (disabled) test code for using OpenImageIO in imbuf
* link cycles, openimageio and boost into blender instead of a shared library
* some cmakefile changes to simplify the code and follow conventions better
* this may solve running cycles problems on windows XP, or give a different
and hopefully more useful error message