last workers take longer when doing parallel rendering

[justusmattern27]

When doing parallel rendering, the workers handling later part of the scenes take longer than the early ones (e.g. worker 0 takes much longer than worker 20). This is likely because of the way we use generator functions for scenes, which makes it more expensive to skip to later parts of a video. Two possible things we can look at:

1. Reduce the time it takes to skip
2. Give user the option to let later workers handle fewer frames than early workers