table-layout: Fix size request when there are no visible rows/cols

The calculation (n - 1) * spacing to compute the total spacing is
only correct for n >= 1 - if there are no visible rows/cols, the
required spacing is 0 rather than negative.

https://bugzilla.gnome.org/show_bug.cgi?id=709434
table-layout: Base space calculations on visible children

This is what we already do in the actual size requests, it makes
sense to do the same in the space calculations.

https://bugzilla.gnome.org/show_bug.cgi?id=709434