Round up ideal width/height calculations so no control is cut off

[carolhmj]

Related forum issue: https://forum.babylonjs.com/t/gui-adapt-tochildren-using-rounded-values-adaptive-scaling/29333

[bjsplat]

Please make sure to label your PR with "bug", "new feature" or "breaking change" label(s).
To prevent this PR from going to the changelog marked it with the "skip changelog" label.

[bjsplat]

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