Meshlet builder improvements redux (#15886)

Take a bunch more improvements from @zeux's nanite.cpp code.

* Use position-only vertices (discard other attributes) to determine
meshlet connectivity for grouping
* Rather than using the lock borders flag when simplifying meshlet
groups, provide the locked vertices ourselves. The lock borders flag
locks the entire border of the meshlet group, but really we only want to
lock the edges between meshlet groups - outwards facing edges are fine
to unlock. This gives a really significant increase to the DAG quality.
* Add back stuck meshlets (group has only a single meshlet,
simplification failed) to the simplification queue to allow them to get
used later on and have another attempt at simplifying
* Target 8 meshlets per group instead of 4 (second biggest improvement
after manual locks)
* Provide a seed to metis for deterministic meshlet building
* Misc other improvements

We can remove the usage of unsafe after the next upstream meshopt
release, but for now we need to use the ffi function directly. I'll do
another round of improvements later, mainly attribute-aware
simplification and using spatial weights for meshlet grouping.

Need to merge #15846 first.

Author: JMS55

Cargo.toml

@@ -1213,7 +1213,7 @@ setup = [
     "curl",
     "-o",
     "assets/models/bunny.meshlet_mesh",
-    "https://raw.githubusercontent.com/JMS55/bevy_meshlet_asset/167cdaf0b08f89fb747b83b94c27755f116cd408/bunny.meshlet_mesh",
+    "https://raw.githubusercontent.com/JMS55/bevy_meshlet_asset/8483db58832542383820c3f44e4730e566910be7/bunny.meshlet_mesh",
   ],
 ]