This change from Aras further parallelizes wihin large meshes (the previous one
just parallelized over objects).
Some stats: on A Windows machine, AMD Ryzen (32 threads):
(one mesh) Monkey subdivided to level 6: 4.9s -> 1.2s (blender 3.1 was 6.3s; 3.0 was 49.4s).
(one mesh) "Rungholt" minecraft level: 8.5s -> 2.9s (3.1 was 10.5s; 3.0 was 73.7s).
(lots of meshes) Blender 3 splash: 6.2s -> 5.2s (3.1 was 48.9s; 3.0 was 392.3s).
On a Linux machine (Threadripper, 48 threads, writing to SSD):
Monkey - 5.08s -> 1.18s (4.2x speedup)
Rungholt - 9.52s -> 3.22s (2.95x speedup)
Blender 3 splash - 5.91s -> 4.61s (1.28x speedup)
For details see patch D14028.
9261bc9476