- TUI: _never_ rely on BCE for implicit clearing, only explicit commands.
- TUI: use unibi_erase_chars when possible.
- TUI: use end-exclusive ranges for invalid and cleared areas
- screen: scrolling leaves scrolled in aree undefined. This is a
conservative change, a client assuming the old semantics will still
behave correctly.
- screen: factor out vsep handling from line drawing. This is needed
anyway for the multigrid refactor.
- screen: simplifications of win_do_lines