Blender Git Loki

Cycles: Fix possible missing updates in PathTrace

The path tracer schedules multiple samples to render to keep device
busy between updates. This was causing an issue with a time check in
the update function: it is possible that N samples finished rendering
slightly faster than aimed 1 second, so the update callback will not
be called. So perceptually rendering was looking sluggish in some
scenes, especially at the lower number of samples rendered.

Now the frequency of updates is fully under PathTrace::render_samples
control.

Technically, simple scenes might be rendering slower now, since there
will be more updates now. This could be addressed by aiming for a
longer render times between updates (schedule more samples to the
device) after reaching certain noise level.