Wrong byteOffset when exporting to GLTF.

When reading triangles from STL and then writing the result to GLB file, depend on indices amount the wrong byteOffset is calculated. As a result some of the libraries assemble wrong positions of vertices.
I've created a repo where the problem is reproduced (see the export.py script): https://github.com/dperyel/open3d-gltf-bug
The repo contains an STL file which will be incorrectly exported.
After running a glb validator I got a report with the errors, you can see them in here: https://github.com/dperyel/open3d-gltf-bug/blob/master/out_report.json

- Operating system: OS, Win, Ubuntu
- Open3d version: 0.13.0 installed with both pip and conda
- Python version: 3.8