Add in-place updates for fixed-topology tensor line sets

[mstoelzle]

This PR is based directly on main and can be reviewed and merged independently of #7570. The shared API additions are arranged consistently with #7570; a synthetic merge of the two branch tips completes without conflicts.

Type

• Bug fix (non-breaking change which fixes an issue): Fixes #
• New feature (non-breaking change which adds functionality). Related to How to update TriangleMesh geometry in Scene? #5541 and Add update_geometries function for gui.SceneWidget #6643
• Breaking change (fix or feature that would cause existing functionality to not work as expected) Resolves #

Motivation and Context

The Filament renderer can add t::geometry::LineSet objects, but applications currently have to remove and re-add them whenever point positions or line colors change. That recreates GPU resources during interactive visualization and simulation.

This change adds in-place updates for fixed-topology tensor line sets.

Checklist:

• I have run python util/check_style.py --apply to apply Open3D code style to my code.
• 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.
• I will follow up and update the code if CI fails.
• For fork PRs, I have selected Allow edits from maintainers.

Description

• Adds Scene::UpdateGeometry overloads for t::geometry::LineSet in C++ and Python, and exposes the same path through Open3DScene and O3DVisualizer.
• Reuses the existing Filament vertex and index buffers for point-position and line-color updates.
• Shares the buffer packing path between initial creation and updates for all three existing layouts: indexed uncolored thin lines, expanded per-line-colored thin lines, and expanded triangle-based wide lines.
• Updates both the current endpoint and partner-endpoint attributes required by the wide-line shader.
• Records source counts, render layout, and normalized line-index hash when the line set is added. Changes to topology, width mode, or an incompatible thin-line color layout are rejected before buffers are modified.
• Refreshes the Filament object bounds and aggregate Open3DScene bounds after point updates.
• Normalizes non-CPU and non-contiguous tensors before asynchronous Filament uploads, whose descriptors retain ownership until the render thread has consumed them.
• Adds an animated tensor LineSet example and documents the fixed-topology contract.
• Covers all three buffer layouts, repeated asynchronous uploads, bounds, and rejection of topology and color-layout changes. The wide-line tests compare the updated rendering byte-for-byte with a remove-and-readd reference, which verifies both endpoint-partner and color buffer updates.

Test results

Tested on Linux x86-64 with GCC 15.2 and Mesa llvmpipe software Vulkan:

TensorGeometryUpdateTest.*
[  PASSED  ] 3 tests.

python/test/visualization/test_tensor_lineset_rendering.py
1 passed

The changed Open3D library, Python binding, and C++ test executable compiled and linked successfully. The local build needed the existing workaround for the unrelated bundled curl/BoringSSL archive-order issue tracked in #7556; that workaround is absent from this branch.

Windows was not available for runtime testing. The implementation uses the existing cross-platform Filament buffer API and retains the current MSVC header guards. Open3D's Windows CI should verify the branch before merge.

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