a step towards solving align

I just realized that with the current formulation of the grid rules,
we can promise that the sum of the columns of a parent are always *at
least* as wide as the parent itself.  Sometimes they're larger (due to
minwidths), but never smaller.

This means that the alignment of a box has no effect on the
alignment/size of the columns.  So we can do alignment as a completely
separate step *after* sizing has finished!

  - a

-- 
"It's lucky," he added, after a pause, "that there are such a lot of
islands in the world.  I almost envy you, Mr. Watson."

                                                   -- Mustapha Mond