UI: restore 'writedelay' feature.

Removed by e0e41b30c6, probably
unintentionally. Useful for ye olde redraw debugging.

TODO: Also delay redraw of statusline.

src/nvim/README.md

@@ -90,6 +90,12 @@ Record a Nvim terminal session and format it with `vterm-dump`:
 
 Then you can compare `bar` with another session, to debug TUI behavior.
 
+### TUI redraw
+
+Set the 'writedelay' option to see where and when the UI is painted.
+
+    :set writedelay=1
+
 ### Terminal reference
 
 - `man terminfo`

src/nvim/ui.c

@@ -453,6 +453,13 @@ void ui_puts(uint8_t *str)
       ui_linefeed();
     }
     p += clen;
+
+    if (p_wd) {  // 'writedelay': flush & delay each time.
+      ui_flush();
+      assert(p_wd >= 0
+             && (sizeof(long) <= sizeof(uint64_t) || p_wd <= UINT64_MAX));
+      os_delay((uint64_t)p_wd, false);
+    }
   }
 }