Margin notes can appear early if page is stretched

[shicks]

If a margin note appears in a paragraph but the page builder decides to instead stretch the glue and ultimately breaks before the paragraph, the margin builder can get confused and try to set the note on the previous page. This can be addressed by (a) clearing the margin once we see a note whose callout position is bigger than the total height of the page, and then (b) re-adding it to the margin list with a non-zero position (as opposed to overflowed margins that get re-added at zero).

A secondary improvement would be to detect the presence of stretch and then scale all the callout positions accordingly.

It's possible we already handle this and I just forgot about it, but it doesn't look like it from a quick glance.

[highplainsscripter]

👍 for addressing this issue.

[RalfJung]

Could this explain why in my situation, the fix to #15 didn't really fix the problem? The sidenotes are rendered now, but on the wrong page. I also see others sitting several lines above the point in the text where they are referenced.