Screenshot with the denoiser produces artifacts

When capturing a screenshot with a resolution higher than the render view size with OSPRay and the denoiser enabled, artifacts are visible at the seam of tiles.
The issue is probably related to pixels of edges not aware of their neighbors.
We should probably execute the denoiser on the full picture instead of each tile independently.

Here is an example (look at the center of the image):

@demarle @carson