Blender Git Loki

Cycles: Write denoising feature passes

With this commit, the newly added passes finally get some content.
The three explicitly stored features are the surface normal, the albedo of the surface
and the path length from the camera. These features will be used by the denoiser to "understand"
where differences in lighting come from - for example, the normal pass allows the denoiser to
smooth the noise on a wall, but keep the edge in the corner of the room perfectly sharp.
To preserve small detail like bumpmapped reflections, the actual normal used for shading
is stored instead of the surface normal which is used for the regular Normal pass.
The main purpose of the albedo pass is to preserve fine texture detail, but can also help to detect
object borders.
The depth pass helps for some edges where both surfaces have the same orientation
(so that normal don't help), but its variance also helps to detect depth-of-field blurring.

The real image passes aren't stored yet because they still require a bit of refactoring.