The actual height and depth of the row is in state.H[i] and state.D[i], but we were using LH and LD, the usual line height and depth. This PR fixes that. This works if there is actual text in the cell, but if the cell is empty, the margin-bottom doesn't fall below the baseline, it sits on top of the baseline, which spoils the alignment. Also, if the height of the cell is less than the midpoint of the font, the browser adds extra height making it the midpoint of the font, and we didn't compensate for that.
Both issues are resolved by adding a 1em strut to the cell (so it has content and is not too short), and compensating for that in the height adjustments. (Since the font in question was the surrounding font, it was not easy to tell where its midpoint was; this solution is independent of the surrounding font.)
Resolves issue #1500 (closed).