Blender Git Loki

Cycles Denoise: Modify TileManager to allow multiple tasks per tile

This commit changes the TileManager and Session so that one tile can be processed
in multiple steps, here rendering and denoising. The reason for that is that a tile
can only be denoised as soon as all its neighbors are rendered (since the filter window
will cross tile borders) and only be freed as soon as all its neighbors are denoised
(otherwise they wouldn't have the out-of-tile data either).
Therefore, Tiles now have a State and are actually stored for the whole render process.
Tile Highlighting also needed a bit of tweaking to un-highlight tiles between rendering
and denoising.

Cycles Denoising: Change PATH_TRACE task to more flexible RENDER task

This commit renames the PATH_TRACE task to RENDER and adds subtypes to the RenderTile, for now
PATH_TRACE and DENOISE. The reason for doing this instead of simply adding a new DENOISE
task is that it 1. allows to reuse the acquire_tile system etc. and 2. allows to denoise tiles
while others are still rendering (if DENOISE was an own task, it would have to wait until PATH_TRACE
was running out of tiles).
The task isn't used yet, that's for the upcoming commits.