Add WSModel and VariantMeshDefinition support to glTF/GLB export

[Shazbot]

Summary

Adds WSModel and VariantMeshDefinition support to the existing glTF exporter.

Changes

• Resolves WSModel material overrides, including sibling WSModels for RMV2 files.
• Exports a composed VMD scene with skeleton attachments.
• Adds animation selection to the export window.
• Supports both .gltf and .glb.
• Fixes swapped DDS colour channels and mask handling.
• Removes temporary embedded texture files after GLB export.
• Adds documentation and integration tests.

Testing

• Full Windows test suite passes.
• Tested RMV2, WSModel, and VMD exports manually.
• Tested VMD export with multiple animations.
• Added real-asset tests using the Karl Franz and Throt fixtures.

Limitations

• Only LOD0 is exported.
• VMD export uses one default composition.
• Warhammer mask textures remain separate PNG files.
• Components using a different skeleton are exported without skinning.