patch 8.1.1443: popup window padding and border not implemented yet

Problem:    Popup window padding and border not implemented yet.
Solution:   Implement padding and border.  Add core position and size to
            popup_getpos().
diff --git a/src/structs.h b/src/structs.h
index 9589b67..fb27745 100644
--- a/src/structs.h
+++ b/src/structs.h
@@ -2888,6 +2888,8 @@
     int		w_maxwidth;	    // "maxwidth" for popup window
     int		w_wantline;	    // "line" for popup window
     int		w_wantcol;	    // "col" for popup window
+    int		w_popup_padding[4]; // popup padding top/right/bot/left
+    int		w_popup_border[4];  // popup border top/right/bot/left
     varnumber_T	w_popup_last_changedtick; // b:changedtick when position was
 					  // computed
     callback_T	w_filter_cb;	    // popup filter callback