Fix table vertical alignment (middle, bottom)
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Fixes table cell vertical alignment (middle, bottom, not yet baseline) when the row contains cells of differing heights.
Moved the work done earlier by @notriddle into a separate public function on TableCellFlow. This function is then called by TableRowFlow once the cell's block size has been calculator.
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- [X] These changes fix#12531 (github issue number if applicable).
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