Fix coordinate indexing in frustum face culling

[tandede]

Summary

Fixes the coordinate indexing used by frustum face culling.

face_verts has shape [F, 3, 3], where the last two dimensions are vertex and xyz coordinate respectively. _get_culled_faces previously indexed it as face_verts[:, axis], which selected one vertex from every face instead of selecting the requested coordinate from all three vertices. The subsequent reduction could therefore miss faces fully outside a clipping plane and cull faces that only had one outside vertex.

This change selects the coordinate with face_verts[:, :, axis] and uses all(dim=1) to express the intended condition directly: a face is culled only when all three vertices lie outside the same frustum plane.

The regression test covers all six frustum planes and verifies that a face intersecting the left plane stays visible for three different vertex orders.

Fixes #1936.

Validation

• The focused regression test passes with all 9 subcases; the pre-fix implementation fails 7 of them.
• An independent oracle over 4,096 seeded random triangles has zero mismatches with the fix. The previous implementation produces 604 false positives and 603 false negatives on the same inputs.
• Black, usort, flake8, and git diff --check pass for the changed files.
• The remaining tests in tests/test_render_meshes_clipped.py require CUDA and cannot run in the local CPU-only PyTorch environment; the focused regression is CPU-only, and the repository CI will cover the CUDA test matrix.

[meta-codesync]

@bottler has imported this pull request. If you are a Meta employee, you can view this in D117200371.