patch 8.1.1918: redrawing popups is inefficient

Problem:    Redrawing popups is inefficient.
Solution:   Fix the logic to compute what window lines to redraw.  Make it
            work below the last line.  Remove redrawing all windows.

src/version.c

@@ -761,6 +761,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    1918,
 /**/
     1917,
 /**/