fix(GltfWriter): emit 32-bit glTF indices for models with >65535 vertices

[vladtrc]

Problem

GltfWriter always writes face indices as unsigned short and hardcodes the index accessor componentType to UNSIGNED_SHORT, while the source field XModelFace::vertexIndex is 32-bit unsigned. All primitives of a model share a single global vertex buffer (positionAccessor.count == xmodel.m_vertices.size()), so for any XModel whose vertex buffer exceeds 65535 vertices every index >= 65536 is silently truncated modulo 65536. The exported glTF then references the wrong vertices and the mesh is corrupted, with no error emitted.

Affected sites in GltfWriter.cpp: the index bufferView byteLength, the index accessor componentType, the static_cast<unsigned short> index write in FillBufferData, and GetExpectedBufferSize.

Why it's real

• XModelFace::vertexIndex is unsigned (32-bit); the writer narrows it to unsigned short.
• static_cast<unsigned short>(N) == N mod 65536, so a face referencing vertex 70000 references vertex 4464 instead — deterministic.
• glTF 2.0 permits componentType 5125 (UNSIGNED_INT) for indices; standard, no extension required.
• Concrete data point: a merged map model reaches 235,386 vertices in a single buffer; under the current code all of its index sets are emitted as UNSIGNED_SHORT, so ~170k vertices are unaddressable. Individual large XModels in the later supported titles hit the same ceiling through the stock xmodel glTF dumper.

Fix

Select unsigned int / UNSIGNED_INT and the matching stride/buffer size when m_vertices.size() > 0xFFFF, keep the existing unsigned short path otherwise. LhcToRhcIndices is templated so the winding conversion stays a single source of truth for both index widths.

Test

Adds GltfWriterTest.cpp: builds a 70000-vertex XModelCommon with a face referencing vertex 69999, runs the writer through a mock gltf::Output, and asserts the index accessor is UNSIGNED_INT (5125) and the high index survives in the buffer unwrapped. Verified against the current tree: the test fails on the unpatched writer (componentType is 5123) and passes with the fix; the full ObjWritingTests suite stays green.

Disclosure

This bug was found, and this change prepared, with heavy use of AI agents while building an independent IW4 asset reader. The analysis, the byte-level reasoning and the vertex counts above are real and were verified against actual game data and the current source — nothing is fabricated. Happy to adjust to your preferred style or add/adapt the regression test.