patch 8.1.1751: when redrawing popups plines_win() may be called often

Problem:    When redrawing popups plines_win() may be called often.
Solution:   Pass a cache to mouse_comp_pos().

src/version.c

@@ -777,6 +777,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    1751,
 /**/
     1750,
 /**/