Blender Git Loki

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.