screen.c: make negative 'writedelay' show all redraws

Currently writedelay shows the sequence of characters that are sent to
the UI/TUI module. Here nvim has already applied an optimization: when
attempting to put a char in a screen cell, if the same char already was
there with the same attributes, UI output is disabled. When debugging
redrawing it it sometimes more useful to inspect the redraw stream one
step earlier, what region of the screen nvim actually is recomputing
from buffer contents (win_line) and from evaluating statusline
expressions.

Take the popupmenu as an example. When closing the popupmenu (in the
TUI), currently 'writedelay' looks like vim only is redrawing the region
which the pum covered. This is not what happens internally: vim redraws
the entire screen, even if only outputs the changed region.

This commit allows negative values of 'writedelay', which causes a delay
for all redrawn characters, even if the character already was displayed
by the UI before.

runtime/doc/options.txt

@@ -6922,7 +6922,8 @@ A jump table for the options with a short description can be found at |Q_op|.
 'writedelay' 'wd'	number	(default 0)
 			global
 	The number of milliseconds to wait for each character sent to the
-	screen.  When non-zero, characters are sent to the terminal one by
-	one.  For debugging purposes.
+	screen.  When positive, characters are sent to the UI one by one.
+	When negative, all redrawn characters cause a delay, even if the
+	character already was displayed by the UI.  For debugging purposes.
 
  vim:tw=78:ts=8:ft=help:noet:norl:

src/nvim/screen.c

@@ -4361,7 +4361,8 @@ static int char_needs_redraw(int off_from, int off_to, int cols)
                       && comp_char_differs(off_from, off_to))
                   || ((*mb_off2cells)(off_from, off_from + cols) > 1
                       && ScreenLines[off_from + 1]
-                      != ScreenLines[off_to + 1])))));
+                      != ScreenLines[off_to + 1])))
+          || p_wd < 0));
 }
 
 /*

src/nvim/ui.c

@@ -437,9 +437,8 @@ void ui_puts(uint8_t *str)
 
     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);
+      uint64_t wd = (uint64_t)labs(p_wd);
+      os_delay(wd, false);
     }
   }
 }