Push the top margin of floats past all useless fragmentainers.
The top margin of a float is not to be split across fragmentainer boundaries if
it can be avoided. We had code to push the margin over to the next
fragmentainer if we were out of space, but we may actually have to push it all
the way to the next fragmentainer *group* (i.e column row) in some cases.
calculatePaginationStrutToFitContent() helps us get there. That's the method we
use to push oversize content (lines and unbreakable blocks) to a better place,
so let's use it for margins too.