Blender Git Loki

Cycles Denoising: Remove tile overscan when using GPU rendering

Denoising a pixel requires access to the other pixels surrounding it. On the CPU, this is solved by waiting for the neighboring tiles to be rendered before the central tile is denoised.
On the GPU, it was handled by rendering larger tiles internally and discarding the overscan area after denoising. That saved a bit of memory, but wasted computation (with 256x256 tiles and a half-window of 8, 13% of rendered pixels were never actually seen).

Also, supporting overscan tiles made the code more complex. So, this commit removes the overscan code and uses the CPU approach on GPUs as well.