While using automatic border locking usually works very well, it
prevents simplifying open geometric borders that the source mesh might
contain. When a mesh has many edges like this, this negatively affects
the DAG quality by locking edges contained within one cluster group.

Instead, we can simply mark vertices that are shared by more than one
group after a partition and feed that data to simplifier.

The effect of this is situational; it's beneficial overall but because
the rest of the pipeline has odd issues with partitioning, even when
using METIS for everything, sometimes this ends up constraining the
build process a little more for unclear reasons. Theoretically this
should be strictly better though, and on some meshes like kitten.obj it
does measurably help, however it needs fixes for attribute seams to
keep it enabled.

This is helpful to estimate the memory impact of the full LOD chain and
makes it easier to compare stats with UE which reports them in log file.

It is pretty difficult to debug the visualized data with just groups
without a sense as to where the cluster boundaries lie; this outputs
duplicate geometry but makes it easier to work with in Blender.

Using 4 clusters is fairly limiting wrt partition quality, as it demands
very careful grouping that METIS is not willing to provide. Instead use
a larger group size (8 atm), and adjust the simplification condition to
be adaptive to the group size so that partitioning is free to produce
groups of varying sizes.

Previously when we built adjacency between groups we would not create
edges between clusters when they shared the position of the vertex with
a different attribute. This meant that when the cluster boundary
contained a crease, it would not be reflected correctly in the
adjacency.

Similarly, but more importantly, we need this information when locking
the cluster boundary for the same reason; before, lockBoundary could
result in gaps because the locking information on the boundary was
inconsistent. This allows us to enable kUseLocks by default.

Instead of using shared edges to provide adjacency, we now use shared
vertices; this works more or less the same way but is easier to compute.
For some reason this also results in fewer stuck triangles when using
full METIS pipeline.