Improve sprite ordering speed

[mockersf]

Objective

• Sprite z ordering can be slow with many sprites

Solution

• use partial_cmp instead of cmp through FloatOrd
• use sort_unstable_by instead of sort_by

results on my laptop:

fps	bevymark -- 25000 40	many_sprites
base	9.84	33.75
partial_cmp	10.44	44.60
sort_unstable_by	10.27	59.96
both	10.59	66.03

the screenshots!

base

partial_cmp

sort_unstable_by

both

[Davier]

I also find that using sort_unstable_by improves performance.
The render items are sorted again in sort_phase_system, but I think that should not become an unstable sort to avoid breaking batches.
The change means that sprites with a depth of NaN may be reordered differently in sort_phase_system, but that's fine by me.

[inodentry]

The logic with the index indirection seems incorrect.

[mockersf]

turns out keeping a smaller index is slower when done without a bug, I removed it

[mockersf]

this will change quite a bit with / is already part of #3460