Enforce glTF index contract on decoded indices

[bjornblissing]

Description

copyDecodedIndices (CesiumGltfReader/src/decodeDraco.cpp) used pIndicesAccessor->componentType to size the decoded index buffer and to pick the copy branch, without validating it first. If the accessor declared a componentType that isn't a legal glTF index type (e.g. BYTE, SHORT, FLOAT, or some other unrecognized value), computeByteSizeOfComponent() returned 0, so the index buffer was sized to zero bytes while the accessor still advertised a nonzero count, and the copy switch matched no case, silently copying nothing into that zero-length buffer — leaving behind an inconsistent, corrupted accessor.

Per the glTF spec, mesh.primitive.indices requires the accessor to have SCALAR type and an unsigned integer componentType, and getIndexAccessorView additionally rejects normalized accessors. This PR narrows the accepted componentTypes to exactly UNSIGNED_BYTE, UNSIGNED_SHORT, and UNSIGNED_INT:

• An illegal componentType is now replaced with one derived from the decoded Draco point count (with a warning), instead of falling through to a zero-sized buffer.
• A legal but too-narrow componentType is still widened as before.
• type is forced to SCALAR and normalized to false, matching the spec requirements.
• The final copy switch now only handles the same three validated types, with the default case asserting unreachability, so the validated set and the copied set can no longer drift apart.

Issue number or link

N/A

Author checklist

• I have submitted a Contributor License Agreement (only needed once).
• I have done a full self-review of my code.
• I have updated CHANGES.md with a short summary of my change (for user-facing changes).
• I have added or updated unit tests to ensure consistent code coverage as necessary.
• I have updated the documentation as necessary.

Testing plan

Steps to reproduce the original issue:

1. Load a glTF model with a KHR_draco_mesh_compression extension where the primitive's indices accessor declares a componentType that is not a legal glTF index type (e.g. FLOAT or a signed integer type).
2. Before the fix, computeByteSizeOfComponent() returns 0 for that componentType, so the decoded index buffer is allocated with zero bytes while the accessor's count remains nonzero, and no data is copied into it — producing a corrupted, inconsistent accessor.
3. After the fix, the illegal componentType is detected and replaced with one derived from the decoded Draco point count, a warning is emitted, and the index buffer is correctly sized and populated.