Fix sparse indices and values, they are objects not arrays

[victor-Lopez25]

I'm slowly doing the glTF 2.0 Khronos tutorial to render glTF and I found sparse accessors were not parsed correctly, so I fixed it. Here are the sources in the glTF 2.0 spec which say the sparse.indices and sparse.values are objects:

• https://registry.khronos.org/glTF/specs/2.0/glTF-2.0.html#reference-accessor-sparse-indices
• https://registry.khronos.org/glTF/specs/2.0/glTF-2.0.html#reference-accessor-sparse-values

and here is the example I found was failing:
https://github.khronos.org/glTF-Tutorials/gltfTutorial/gltfTutorial_005_BuffersBufferViewsAccessors.html#sparse-accessors

I haven't gotten to rendering it yet though 😅 that might be a bit harder.

[victor-Lopez25]

There might be a similar fix needed in glb parsing but I haven't checked.