Performance: Divvy up regions of the image in the same image to multiple workers

[evelynhathaway]

Along the direction, we can split the workload of sorting. As long as no intervals are split, sorting results can be combined at the end.

This can be done by, if sorting vertically, take a certain amount of vertical strips and send that to another worker. Or potentially, just send intervals if the intervals can performantly be made in one worker in less time than it takes to serialize the entire image data into pieces.