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().
diff --git a/src/evalfunc.c b/src/evalfunc.c
index 73efa79..685e2f1 100644
--- a/src/evalfunc.c
+++ b/src/evalfunc.c
@@ -4738,7 +4738,7 @@
 		win = mouse_find_win(&row, &col, FIND_POPUP);
 		if (win == NULL)
 		    return;
-		(void)mouse_comp_pos(win, &row, &col, &lnum);
+		(void)mouse_comp_pos(win, &row, &col, &lnum, NULL);
 # ifdef FEAT_TEXT_PROP
 		if (WIN_IS_POPUP(win))
 		    winnr = 0;