vim-patch:9.0.0318: clearing screen causes flicker (#19993)

Problem:    Clearing screen causes flicker.
Solution:   Do not clear but redraw in more cases.  Add () to "wait_return".
13608d851a

Only 2 lines of actual code change.

src/nvim/getchar.c

@@ -1768,7 +1768,7 @@ static void getchar_common(typval_T *argvars, typval_T *rettv)
 
   if (!ui_has_messages()) {
     // redraw the screen after getchar()
-    update_screen(UPD_CLEAR);
+    update_screen(UPD_NOT_VALID);
   }
 
   set_vim_var_nr(VV_MOUSE_WIN, 0);