gltfpack: Keep EXT_mesh_gpu_instancing

[zeux]

Discussed in #680

^{Originally posted by BoyBaykiller April 22, 2024}
I am running gltfpack -v -mi -noq -tc -i .\SimpleInstancing.gltf -o .\Compressed\Output.gltf over this gltf model.
It uses the EXT_mesh_gpu_instancing extension to define a bunch of instanced cubes. I can render it correctly as it, but after running it through the command the EXT_mesh_gpu_instancing extension is removed, resulting in only a single cube being rendered.