Optimize "Dynamic bucket splitting" function

[prorender]

There are cases where "Dynamic bucket splitting" in DR finishes the rendering before some single buckets have finished. Sometimes the rendering of these last buckets takes much time. So it would be a optimization to find these last buckets and replace them also with the "Dynamic bucket splitting" function.

[Vizioen]

Has been asked a few times already:

https://forums.chaosgroup.com/forum/...ket-time-limit
https://forums.chaosgroup.com/forum/...cket-splitting
https://forums.chaosgroup.com/forum/...g-improvements

It's possible only hasn't reached us yet

[^Lele^]

lose faith, and be surprised, or be patient and expect the cool. :P