Previously, positioning such an item would hang while trying to find an
appropriate position for the item, because no position in the grid was
suitable, and implicit cells in the layout direction would be added
until a viable position was found.

We now ensure that there are enough cells in the cross direction to hold
each of the auto-placement items before trying to position those items.

Negative indices should count backwards from the last explicit
row/column number. If the resulting line number would appear before
the first explicit row/column, implicit grid lines should be added
before the first explicit row/column.