EXT_mesh_gpu_instancing: Suggest extensionRequired usage

[zeux]

Using this extension without specifying it as required is guaranteed to lead to significant differences in appearance between renderers that support it and renderers that don't due to how it is specified.

Fixes #2402.

[bghgary]

There are two sample models in this folder. Should we update it with extensionsRequired?

https://github.com/KhronosGroup/glTF/blob/main/extensions/2.0/Vendor/EXT_mesh_gpu_instancing/samples/teapots_galore/teapots_galore.gltf#L286-L288

https://github.com/KhronosGroup/glTF/blob/main/extensions/2.0/Vendor/EXT_mesh_gpu_instancing/samples/teapots_galore_id/teapots_galore_id.gltf#L320-L322

EDIT: Should these models be in https://github.com/KhronosGroup/glTF-Sample-Assets instead of in the extension folder?

[zeux]

There’s an instancing example (SimpleInstancing) in glTF- Sample-Assets so I don’t know if we need a second one; not sure what the policy is wrt having examples in this repository. I plan to submit a PR for SimpleInstancing if this gets merged but I didn’t realize there are examples in this repository as well…

[bghgary]

@zeux Sorry, my question isn't directed at you specifically, though you are welcome to comment also of course. Maybe @lexaknyazev and others can chime in.

not sure what the policy is wrt having examples in this repository

I'm not sure either. I think this might be the only extension with sample assets inside the extension folder itself.

[lexaknyazev]

That's 100% an oversight. Samples should be in the dedicated repo.

[javagl]

I agree that the models should not be here. (Moving them to glTF-Sample-Assets will require some work (just to make clear that it's not CTRL-X/V), and ... I think that only the one with the _ID should be enough...)

[zeux]

Please let me know if any further changes are needed on this PR (or if there are no plans to merge it).