Currently Flamenco uses uniform sample chunks, so if you want to have a result with only 1 sample, it has to create tasks for each individual sample. What we want is something like:

- render with 1 sample
- render with 9 additional samples to get 10 samples
- render with 40 additional samples to get 50 samples
- render wtih 50 additional samples to get 100 samples

and so on. It'll probably be okay to use some exponential distribution and pick the number of samples from that. So if the user requests 1000 samples in 5 chunks, use:

- 1 sample
- 6 samples
- 32 samples
- 178 samples
- 1000 samples

Alternatively, a quadratic distribution would result in:

- 1 sample
- 63 samples
- 251 samples
- 563 samples
- 1000 samples

@Andy Goralczyk (eyecandy), @venomgfx, which distribution (exponential or quadratic) would you prefer?

PS: this functionality is made possible by dr. @Sergey Sharybin (sergey)'s awesome work in rBc5dba54 and rBc44cdd5