Fix stack panel giving a warning for children with resizeToFit.

[carolhmj]

Forum issue: https://forum.babylonjs.com/t/control-is-using-height-in-percentage-mode-inside-a-vertical-stackpanel-warnings-after-update-to-6-25-0/45071

[bjsplat]

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[bjsplat]

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[bjsplat]

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[bjsplat]

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[bjsplat]

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[bjsplat]

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