Add preliminary GPU instance metadata example

[javagl]

A PRELIMINARY state of an example that uses EXT_mesh_gpu_instancing and EXT_structural_metadata.

The sandcastle for accessing the data uses a private API. When there is a public API, this example can be updated.

[lilleyse]

This is a nice surprise!

You should be able to use EXT_instance_features to avoid the private API.

If you feel like taking this further you could add another sample that has multiple feature ID sets

[javagl]

Yes, I also mentioned that at https://community.cesium.com/t/from-i3dm-to-ext-mesh-gpu-instancing/19731/2 (which was the reason for reviving the model that I started back in #40 (comment) ...). I'll try to do the update soon, but will have to do some local updates/extensions (sic) to properly support EXT_instance_features...

[javagl]

The sample has been finalized and added to the main README.

Since it neither belongs to EXT_mesh_features nor to EXT_structural_metadata, but combines multiple extensions, I think it makes sense to put it directly into a subdirectory of the glTF directory. But if desired, the layout can be adjusted.

Entry point: https://github.com/javagl/3d-tiles-samples/tree/add-gpu-instance-metadata-example/glTF#other-gltf-extensions

[lilleyse]

👍