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/ugrid.h

@@ -22,15 +22,13 @@ struct ugrid {
 
 // -V:UGRID_FOREACH_CELL:625
 
-#define UGRID_FOREACH_CELL(grid, top, bot, left, right, code) \
+#define UGRID_FOREACH_CELL(grid, row, startcol, endcol, code) \
   do { \
-    for (int row = top; row <= bot; row++) { \
-      UCell *row_cells = (grid)->cells[row]; \
-      for (int col = left; col <= right; col++) { \
-        UCell *cell = row_cells + col; \
-        (void)(cell); \
-        code; \
-      } \
+    UCell *row_cells = (grid)->cells[row]; \
+    for (int col = startcol; col < endcol; col++) { \
+      UCell *cell = row_cells + col; \
+      (void)(cell); \
+      code; \
     } \
   } while (0)