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/beval.c b/src/beval.c
index 3dd9b67..69e667d 100644
--- a/src/beval.c
+++ b/src/beval.c
@@ -43,7 +43,7 @@
     {
 	// Found a window and the cursor is in the text.  Now find the line
 	// number.
-	if (!mouse_comp_pos(wp, &row, &col, &lnum))
+	if (!mouse_comp_pos(wp, &row, &col, &lnum, NULL))
 	{
 	    // Not past end of the file.
 	    lbuf = ml_get_buf(wp->w_buffer, lnum, FALSE);