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.

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));
 }
 
 /*