patch 8.1.1600: cannot specify highlighting for popup window scrollbar

Problem:    Cannot specify highlighting for popup window scrollbar.
Solution:   Add "scrollbarhighlight" and "thumbhighlight" options.
diff --git a/src/window.c b/src/window.c
index c6d4aa3..d40b060 100644
--- a/src/window.c
+++ b/src/window.c
@@ -4857,6 +4857,8 @@
     free_callback(&wp->w_filter_cb);
     for (i = 0; i < 4; ++i)
 	VIM_CLEAR(wp->w_border_highlight[i]);
+    vim_free(wp->w_scrollbar_highlight);
+    vim_free(wp->w_thumb_highlight);
     vim_free(wp->w_popup_title);
     list_unref(wp->w_popup_mask);
 #endif