Previously, the column count was always incremented by 1. This led to a
mismatch with `compute_outer_content_sizes()`, which did use the `span`
attribute to advance the column index. This mismatch caused an
out-of-bounds access when the column index was greater than the
expected number of columns.
Some of this is rearranged for clarity, but it's mostly the exact same
code. Steps 3, 10, 11, and 15 are new, but don't have any effect until
we implement downward-growing cells.
When a table row (or its group) is set to collapse, the row takes up no
vertical space in the layout.
We have to account for this in multiple places, so I've cached whether a
row is collapsed in the TableGrid::Row.
Before this change, we were always processing all row groups first,
and then all rows. This led to incorrect table layouts when rows and row
groups were encountered in anything but that order.
