Blender Git Loki

Cycles: Speedup adaptive sampling on CPU

- Do early output in the convergence test, to avoid error calculation
for pixel which is known to be converged.

- Better threading scheduling in the path trace work, to avoid extra
call of `parallel_for` which has (unmeasurable) overhead.

- Biggest change is to stop parallel samples for pixel once it did
converge. Prior to this change the path trace work will attempt to
initialize path state for many samples in a row for this pixel.

Timing on sampling simple file (diffuse monkey on diffuse plane):

- master is 3.89 sec
- before this change: 4.24 sec
- after this change: 4.04 sec