Grainsize calculation on TinyThreads #142

I was working fine with TBB in my app on Windows, defining the grainsize, but I got problems when porting to Linux and using TinyThreads. So I looked at the code.

Here I can see that, if I have a vector of size 767, and I set the grainsize to 64 I get the following:

1. Total threads is 8
2. So, 767/8 is not exact, and the algorithm chooses to do 767/(8-1) ~= 109
3. So each chunk will be of size 109. That is fine for my algorithm that needs a chunk larger than, let's say, 60.
4. There will be 767 / 109 = 7.0366 jobs. That means one job will be of size 4 that is below my grainsize of 64. And this will break my algorithm.

So, what about the last chunk to be of size 113 instead of 109 + 4? This will be in conformity with what is expected from grainsize.

I can write a pull request. For now, I'm just discussing the case.