Blender Git Loki

Optimize render part commiting to render queue to mitigate delay in
T44869.

There are a couple of issues here:

* Code repeatedly calculated center of ready rendered parts even though
they would not change while the operation was done.

* Code would calculate distance of tiles from center multiple times

* Code would traverse all items, even the ones already sorted
* Traversal used linked lists which is quite slow.

Mitigated these by doing one pass for the center, a second to calculate
distances and a qsort at the end.

Should result in O (n * (log n + 2)) instead of O (n * (n * 2))
complexity, plus the number of repeated operations is much less as well.