- Only add line spacing between lines. Previously, line spacing was added below
every line, making `#box[word]` higher than just `word`.
- Compute box height of text as `ascender - descender` so that the full word is
contained in the box.
The name run was a relict of the time where a line consisted of a set of runs with same alignment. While these runs still exist conceptually, they are all stored flatly together in what was now renamed from `run` to `line`.
Laurenz