instancedMesh: optimize initialization performance

[kzhsw]

In the constructor of InstancedMesh, refreshBoundingInfo if called, but if the source mesh does not contain skeleton or morphTargetManager, we can use bounding info of source mesh directly, to avoid overheads introduced by refreshBoundingInfo.

Forum post: https://forum.babylonjs.com/t/instancedmesh-why-fully-refreshboundinginfo-even-without-bones/58543

[bjsplat]

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[bjsplat]

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