Blender Git Loki

Cycles: Add temporary debugging environment variable, fix debug file extensions

As explained previously, CPUs currently get the out-of-tile pixels for denoising
from neighbor tiles, while GPUs just render bigger tiles internally.
However, implementation differences can make the GPU version (aka "overscan" rendering)
fail while the CPU code works.

Therefore, this commit adds a environment variable check for whether CPU_OVERSCAN is defined,
and enables the already-present CPU single-tile overscan mode for easier debugging.
Note that this has no benefits at all for regular use and will be removed later!