Previously the layout used the floor of the calculated start and width
as the value to use for the split Rects. This resulted in gaps between
the split rects.
This change modifies the layout to round to the nearest column instead
of taking the floor of the start and width. This results in the start
and end of each rect being rounded the same way and being strictly
adjacent without gaps.
Because there is a required constraint that ensures that the last end is
equal to the area end, there is no longer the need to fixup the last
item width when the fill (as e.g. width = x.99 now rounds to x+1 not x).
The colors example has been updated to use Ratio(1, 8) instead of
Percentage(13), as this now renders without gaps for all possible sizes,
whereas previously it would have left odd gaps between columns.
