The standard `threading::parallel_for` function tries to split the range into
uniformly sized subranges. This is great if each element takes approximately
the same amount of time to compute.
However, there are also situations where the time required to do the work for
a single index differs significantly between different indices. In such a case,
it's better to split the tasks into segments while taking the size of each task into
account.
This patch implements `threading::parallel_for_weighted` which allows passing
in an additional callback that returns the size of each task.
Pull Request: https://projects.blender.org/blender/blender/pulls/118348
9a3ceb79de