`oiiotool` diff generation is single-threaded and can become a bottleneck on fast to render tests. This PR generates diffs in parallel using the `multiprocessing` module. Overlay tests (local): 90s -> 30s buildbot +gpu: macOS: 2020s -> 1808s Linux: 1901s -> 1327s Pull Request: https://projects.blender.org/blender/blender/pulls/134938