Mesh: Make cloning thin instances optional

[Popov72]

See https://forum.babylonjs.com/t/cloning-after-thin-instance-back-compat-issue/55688/16 for context.

Follow up to #15997. It removes the breaking change, as the user will now have to explicitely ask for thin instances cloning.

Original PG fixed with the change from this PR:

https://playground.babylonjs.com/#92Y727#461

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