[css-rhythm] Handle margins at breaks for block-step-sizing

[fantasai]

At unforced page breaks, the margin disappears. So if the style is set to insert additional “margin” when adjusting the height, it should not add to the side that has a truncated margin.

[fantasai]

Fixed in 45d60cd , @dauwhe would you mind reviewing to make sure this is the correct behavior? :)

[dauwhe]

There might be a complication. With headings, often the space below the heading is fixed, but we're free to adjust the space above. The space below is important, because we want to keep the visual connection to the text that the head describes. But when a head occurs at the top of a column, sometimes it needs to be pushed down a bit so that everything below it aligns properly. See latinreq for a very simple illustration.

Here's a screenshot from a little toy example. In this case, the h2 needs 4pt of space above (shown dramatically in red) at the column break. If the heading were two lines it wouldn't need any.