vim-patch:9.0.0656: cannot specify another character to use instead of '@'

Problem:    Cannot specify another character to use instead of '@' at the end
            of the window.
Solution:   Add "lastline" to 'fillchars'. (Martin Tournoij, closes vim/vim#11264,
            closes vim/vim#10963)
4ba5f1dab6

Use latest code in drawscreen.c instead.

src/nvim/buffer_defs.h

@@ -1168,6 +1168,7 @@ struct window_S {
     int diff;
     int msgsep;
     int eob;
+    int lastline;
   } w_p_fcs_chars;
 
   // "w_topline", "w_leftcol" and "w_skipcol" specify the offsets for