Button3D: Allow setting the dimensions at creation time

[Popov72]

See https://forum.babylonjs.com/t/holographicbutton-button3d-sizes/39134/3

The dimensions can be set at creation time only because it has an impact on the creation of the mesh. I would have to recreate the mesh if we allowed the dimensions to be changed at any time, but _createNode (which creates the mesh) is called by Control3D, and the result of that call is stored in a private variable. If the user wants to change the dimensions afterwards, he will have to recreate the button.

[bjsplat]

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[bjsplat]

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