Consistent pagination strut propagation policies.
There's no break opportunity between a spanner and the next block in a column.
Also no break opportunity between a float and an in-flow block. The spec is a
bit vague here, but our implementation certainly doesn't support it.
We have to prevent strut propagation in such cases. If the first piece of
content inside the block (e.g. a line) doesn't fit inside the current
fragmentainer, it's the line that needs to be pushed, not the block.
One of the tests included here used to cause a DCHECK failure.