Don't allow AsciiDoc table cell to overrun bottom of page on which it fits

[mojavelinux]

It's possible for the height reserved for the AsciiDoc table cell to be insufficient due to rounding and estimation errors. As a result, a cell which should fit on a page is truncated and a warning is issue that the cell exceeds the height of the page.

Once the cell is determined to fit on the page, the height of the box in which to draw the content should be equal to the writable area of the page (minus the top and bottom padding of the cell). That way, a warning is only issued if the height of the content truly is greater than the height of the page and any false warnings can be avoided.