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.
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