Update the GLTF writer to support writing textured models

[dbs4261]

The GLTF exporter does not currently support textured meshes. This is because Open3d's per triangle UV coordinates makes the task non-trivial. This change set adds that feature. Note: this depends on #5938 which changes how textures are stored in an open3d::geometry::TriangleMesh

Type

• [ ] Bug fix (non-breaking change which fixes an issue): Fixes #
• [x] New feature (non-breaking change which adds functionality). Resolves #
• [ ] Breaking change (fix or feature that would cause existing functionality to not work as expected) Resolves #

Motivation and Context

The current GLTF exporter does not support textured meshes. This PR adds an exporter for TriangleMeshModels. Currently there is only a reader. It does not add support to the existing writer.

Checklist:

• [x] I have run python util/check_style.py --apply to apply Open3D code style to my code.
• [x] This PR changes Open3D behavior or adds new functionality.
  • [ ] Both C++ (Doxygen) and Python (Sphinx / Google style) documentation is updated accordingly.
  • [ ] I have added or updated C++ and / or Python unit tests OR included test results (e.g. screenshots or numbers) here.
• [x] I will follow up and update the code if CI fails.
• [x] For fork PRs, I have selected Allow edits from maintainers.

Description

Code was added to create TriangleMeshModels from a TriangleMesh. This isn't a simple conversion because TriangleMesh's support multiple materials, assigned on a per triangle basis. However TriangleMeshModels are supposed to only have one material per mesh. There is a similar issue for the export of the meshes. TriangleMesh's store texture coordinates on a per triangle basis. This is to support seams where the same vertex has different texture coordinates. However GLTF requires texture coordinates to be stored on a per vertex basis. So this PR adds a function to convert a mesh to have per-vertex texture coordinates. Currently this function is in a detail namespace but could be moved to a public API.

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