win_line: update w_last_cursorline always

Vim patch 8.1.0856 (54d9ea6) caused a performance regression in Neovim,
when `set conceallevel=1 nocursorline` was used, since then due to
refactoring in 23c71d5 `w_last_cursorline` would never get updated
anymore.

Adds/uses `redrawdebug+=nodelta` for testing this.

Fixes https://github.com/neovim/neovim/issues/11100.
Closes https://github.com/neovim/neovim/pull/11101.

src/nvim/option.c

@@ -4306,6 +4306,10 @@ static char *set_num_option(int opt_idx, char_u *varp, long value,
     if (value < 0) {
       errmsg = e_positive;
     }
+  } else if (pp == &p_wd) {
+    if (value < 0) {
+      errmsg = e_positive;
+    }
   }
 
   // Don't change the value and return early if validation failed.

src/nvim/option_defs.h

@@ -518,11 +518,18 @@ EXTERN long p_pyx;              // 'pyxversion'
 EXTERN char_u *p_rdb;           // 'redrawdebug'
 EXTERN unsigned rdb_flags;
 # ifdef IN_OPTION_C
-static char *(p_rdb_values[]) = { "compositor", "nothrottle", "invalid", NULL };
+static char *(p_rdb_values[]) = {
+  "compositor",
+  "nothrottle",
+  "invalid",
+  "nodelta",
+  NULL
+};
 # endif
 # define RDB_COMPOSITOR         0x001
 # define RDB_NOTHROTTLE         0x002
 # define RDB_INVALID            0x004
+# define RDB_NODELTA            0x008
 
 EXTERN long p_rdt;              // 'redrawtime'
 EXTERN int p_remap;             // 'remap'

src/nvim/screen.c

@@ -577,7 +577,7 @@ void conceal_check_cursor_line(void)
 /// Whether cursorline is drawn in a special way
 ///
 /// If true, both old and new cursorline will need
-/// need to be redrawn when moving cursor within windows.
+/// to be redrawn when moving cursor within windows.
 /// TODO(bfredl): VIsual_active shouldn't be needed, but is used to fix a glitch
 ///               caused by scrolling.
 bool win_cursorline_standout(const win_T *wp)
@@ -2406,10 +2406,10 @@ win_line (
   filler_todo = filler_lines;
 
   // Cursor line highlighting for 'cursorline' in the current window.
-  if (wp->w_p_cul && lnum == wp->w_cursor.lnum) {
+  if (lnum == wp->w_cursor.lnum) {
     // Do not show the cursor line when Visual mode is active, because it's
     // not clear what is selected then.
-    if (!(wp == curwin && VIsual_active)) {
+    if (wp->w_p_cul && !(wp == curwin && VIsual_active)) {
       int cul_attr = win_hl_attr(wp, HLF_CUL);
       HlAttrs ae = syn_attr2entry(cul_attr);
 
@@ -4354,7 +4354,7 @@ static int grid_char_needs_redraw(ScreenGrid *grid, int off_from, int off_to,
                || (line_off2cells(linebuf_char, off_from, off_from + cols) > 1
                    && schar_cmp(linebuf_char[off_from + 1],
                                 grid->chars[off_to + 1])))
-              || p_wd < 0));
+              || rdb_flags & RDB_NODELTA));
 }
 
 /// Move one buffered line to the window grid, but only the characters that