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Cycles X: Multi-device re-balancing

This is an initial implementation which seems to give better
device utilization here when using two non-matched GPUs, as
well as multi-GPU and CPU.

General idea is to balance amount of work based on an
observed performance of devices, and "re-slice" the big tile.

Things which are known to be not final but considered a further
development:

- The balancing algorithm might need some tweaks for the
objective function and weight modification to converge to
the ideal balance quicker.

- The "re-slicing" might also be optimized memory-wise.

- Headless rendering needs to give few iterations of smaller
works to allow multi-device to settle down in the balance.

The balancing logic is in own little file, which simplifies
process of experiments.

Differential Revision: https://developer.blender.org/D11774