This fixes a bug in the CSS Grid when there is a column and/or row gap,
as previously it would take the index of the incorrect column when
finding the `AvailableSize`.
There is a mild complication in the GridFormattingContext as the
OccupationGrid does not take into account the gap columns and rows that
later appear in the `Vector<TemporaryTrack>` columns and rows. The
PositionedBoxes are kind of a connection between the two, and so it's
now more explicit whether you would like to refer to a column by its
position taking into the gap columns/rows or not.
52e45fb6fa
