In addition to the regular edge collapse, we now allow the simplifier to
remove small isolated components. Components that are removed are below
the error threshold in size and as such should not noticeably contribute
to the overall rendering of the object.
This helps simplify topologically complex but small parts of larger
meshes and more comfortably reach the LOD targets.
In some cases, pruning can cause the last LOD to shrink to 0 triangles
which may prevent a slightly larger LOD from being used at the maximum
distance; in this case we retry simplification without pruning once.
ccbece58d8