Pass "allowCanvas" into override serialize functions.

[jamessimo]

Fixes a bug with instances of BabylonJS running in a "Canvas-less" environment crashing when trying to serialize buttons.

Right now, when using the serialize() function in buttons, they ignore the "allowCanvas" parameter which was added in #15033

Link to BabylonJS Forum discussion:
https://forum.babylonjs.com/t/skip-canvas-being-used-during-control-serialize/49302/13?u=jamessimo

[bjsplat]

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[bjsplat]

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[bjsplat]

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[bjsplat]

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