Fix memory usage becomes very large when loading STL files

[adamkewley]

Bug from upstream: simbody/simbody#798

The new normals/texture coordinates implementation does not correctly accumulate normals data during STL loading, which results in an algorithm that adds N1, then (N1,N2), then (N1,N2,N3), and so on for each vertex, which makes the collection grow quadratically and blow up ram (internal tests show (e.g.) that loading a basic model uses 15 GB of RAM).