Add multiple models support

[alexandremottet]

Add support for multiple model on the same scene by :

• adding a private _loadedModelBacking to reference all the model loaded on the scene,
• renaming _modelInfo to _activeModelBacking to represent to current active/selected model
• adding as protected a _addModel and a _removeModel

[bjsplat]

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To prevent this PR from going to the changelog marked it with the "skip changelog" label.

[bjsplat]

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Visualization tests for WebGPU

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[bjsplat]

WebGL2 visualization test reporter:

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