The specification isn't explicit about it, but the contribution we
compute should be distributed to all columns, not just the first one.
The first reason for it is symmetry, it doesn't make sense for the
increased width of the spanning column to only affect the first column
in the span.
The second reason is the formula for the cell contribution, which is
weighted by the non-spanning width of the cell relative to the total
width of the columns in the same row. This only covers a fraction of the
gap, in order to fully cover it we have to add it to all columns in the
span. For this to be exactly the case when the columns don't all have
the same width, we'd have to add additional weighting based on the width
ratios, but given that the specification doesn't suggest it at all we'll
leave it out for now.
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