UI/TUI: improvements and cleanups for scrolling and clearing

- 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

src/nvim/move.c

@@ -918,9 +918,9 @@ void curs_columns(
 
     extra = ((int)prev_skipcol - (int)curwin->w_skipcol) / width;
     if (extra > 0) {
-      win_ins_lines(curwin, 0, extra, false);
+      win_ins_lines(curwin, 0, extra);
     } else if (extra < 0) {
-      win_del_lines(curwin, 0, -extra, false);
+      win_del_lines(curwin, 0, -extra);
     }
   } else {
     curwin->w_skipcol = 0;