patch 8.1.1920: cannot always close a popup when filter consumes all events

Problem:    Cannot close a popup by the X when a filter consumes all events.
Solution:   Check for a click on the close button before invoking filters.
            (closes #4858)
diff --git a/src/proto/popupwin.pro b/src/proto/popupwin.pro
index 0507114..dab117e 100644
--- a/src/proto/popupwin.pro
+++ b/src/proto/popupwin.pro
@@ -1,6 +1,6 @@
 /* popupwin.c */
 int popup_on_border(win_T *wp, int row, int col);
-int popup_on_X_button(win_T *wp, int row, int col);
+int popup_close_if_on_X(win_T *wp, int row, int col);
 void popup_start_drag(win_T *wp, int row, int col);
 void popup_drag(win_T *wp);
 void popup_set_firstline(win_T *wp);