Split tiles by workload

[LZaw]

Having smaller tiles where there is a lot of work to do balances the time spend per tile. The general idea is to do a first very coarse run to estimate the workload per tile and then do the actual render in subsequent runs, splitting tiles by the estimated workload of the first run. Also, for subsequent runs, adapt tile sizes again by the previous run.

It's highly recommended to also implement #1088 when working on this Issue. Possibly #1701 would not be needed anymore after finishing this one.