Draw spaces: fix regression

gtk_text_iter_forward_to_line_end says:
If the iterator is already at the paragraph delimiter characters, moves
to the paragraph delimiter characters for the next line.

Because of this, get_end_iter treated the whole line following a blank
line as "inside text".
Similarly, get_leading_trailing also treated the whole line following a
blank as "trailing".

https://bugzilla.gnome.org/show_bug.cgi?id=730713