add support for custom attributes to convertToUnIndexedMesh

[BlakeOne]

Add support for custom attributes to the function convertToUnIndexedMesh. Currently calling it on a mesh with custom attribute(s) set causes an error to be thrown since the attribute's stride can't be deduced.

Forum: https://forum.babylonjs.com/t/allow-converttounindexedmesh-to-support-custom-attributes/31486

[azure-pipelines]

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[azure-pipelines]

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