Blender Git Loki

Cycles: Add denoise-only mode the the Session

This mode just creates the Device, generates tiles and runs the denoising kernel
on each tile.
Compared to the regular mode of operation, a lot is missing: No interactivity, no
scene syncing, no progressive rendering etc. However, these features aren't needed
for the denoise-after-render feature, and so this mode saves a lot of code when
calling it from the bindings.

Internally, it uses one single large buffer to hold the image, instead of a small buffer
per tile. That requires some changes to the TileManager and is also the reason for the
earlier region-of-interest commit.