Two small bug fixes

We should set the variable _last_tr_data to some default value so the
publisher doesn't complain with an error and raises an exit status > 0
Update documentation on image

still not complete
Enhance split to disallow breaks
Fix vsplit
Don't put "margin-bottom" on top of text

when we end a text with margin-bottom (a glue) and there is not enough
room for that glue, it will end up at the top of the next area. Which
is not what we want, since it's margin-bottom. So we just that item.