Should be the last bit of sculpt/paint code, now this is fully using BLI_task.
Note that PBVH normals update is now about 20% quicker than with OMP code
(from 27ms to 21ms with a big stroke over a 500k vertices monkey), even though
a missing thread-protection was added... atomic primitives ftw!
For the records, with the missing `#pragma omp critical` section added,
previous code was like four times slower (above 100ms).
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