gltfpack: Adjust the aggressive simplification threshold
gltfpack: Adjust the aggressive simplification threshold

Instead of checking the source mesh complexity, we now check the target
mesh complexity - if we're asked to simplify to under 50 triangles the
sloppy simplification results are likely to be too distorted to bother.

zeux/meshoptimizer,zeux/meshoptimizer,zeux/meshoptimizer