Fix calculation of text height in InputTextArea

[carolhmj]

The variables used to calculate where the text's y position would be in the InputTextArea were not using the most up-to-date measure of the control, so the position would be incorrect inside a scroll view:

With the fix:

[bjsplat]

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To prevent this PR from going to the changelog marked it with the "skip changelog" label.

[bjsplat]

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